Matter, Atomic Structure, and Properties

Matter and Mass are two basic ideas in science that help us describe the world around us. Everything you can touch, hold, pour, or breathe is made of matter. Matter is what makes up objects like books, water, air, rocks, and even your own body.

To be called matter, something must have mass and take up space. Taking up space means it has volume. If something has both mass and volume, it is matter.

This lesson will help you understand what matter is, what mass means, how mass is measured, and how matter is different from things like light, heat, and ideas.

What is matter?

Matter is anything that has mass and volume. Mass tells us how much “stuff” is in an object. Volume tells us how much space the object takes up.

For example, a basketball is matter because it has mass and takes up space. A glass of water is matter too. Even air is matter, even though you cannot usually see it, because it has mass and fills space inside a balloon or a room.

Some things are not matter. Light is not matter because it does not have mass in the way objects do, and it does not take up space like a solid, liquid, or gas. Heat, sound, and electricity are forms of energy, not matter. Ideas, feelings, and numbers are also not matter because they do not have mass or volume.

Key idea: If it has mass and takes up space, it is matter.

What is mass?

Mass is the amount of matter in an object. It is one of the main ways scientists describe matter.

Mass is often measured in grams \,\((g)\) or kilograms \,\((kg)\). Small objects, like a paper clip or an apple, are often measured in grams. Larger objects, like a person or a backpack, are often measured in kilograms.

Remember this metric relationship:

$$1\ \text{kg} = 1000\ \text{g}$$

Mass is not the same as size. A small object can have a lot of mass if it is made of a heavy material. A larger object can have less mass if it is made of a lighter material.

For example, a small metal ball may have more mass than a large foam ball. The foam ball looks bigger, but the metal ball contains more matter.

Mass is different from weight

Students sometimes confuse mass and weight. They are related, but they are not the same thing.

• Mass is how much matter is in an object.
• Weight is how strongly gravity pulls on that object.

On Earth, an object with more mass usually also has more weight. But mass stays the same wherever you go, while weight can change if gravity changes.

For example, if you took a rock to the Moon, its mass would stay the same, but its weight would be less because gravity on the Moon is weaker than gravity on Earth.

In 7th Grade science, the most important idea is this: mass measures the amount of matter.

What is volume?

Volume is the amount of space matter takes up. Solids, liquids, and gases all have volume.

A shoe takes up space in a closet. Water takes up space in a bottle. Air takes up space in a balloon. Since all of these take up space and have mass, they are all forms of matter.

We often notice the volume of solids and liquids easily, but gases can be harder to notice. Even though you cannot always see a gas, it still spreads out to fill the space available. That is why air is matter.

States of matter

Matter can be found in different forms called states of matter. The three main states you will study are:

• Solid – has a shape and volume of its own, like a rock or pencil.
• Liquid – has a volume but takes the shape of its container, like water or juice.
• Gas – does not have its own shape or volume and spreads out to fill its container, like air or steam.

All three states are matter because they all have mass and take up space.

How do we know air is matter?

Air is a great example because many students think invisible things are not matter. But air is matter.

If you blow up a balloon, the balloon gets bigger because the air inside takes up space. If you compare an empty balloon and an inflated balloon carefully, the inflated balloon has more mass because of the air added inside it.

This shows that air has both volume and mass, so it is matter.

Matter and non-matter

It is useful to compare matter with things that are not matter.

• Matter: water, milk, wood, air, metal, sand, your desk
• Not matter: light, heat, sound, electricity, thoughts, music, time

Some of these non-matter examples are forms of energy, and some are ideas or ways of measuring things. They may be important, but they do not have mass and do not take up space.

How scientists measure mass

Scientists measure mass with a balance. A balance compares an object to known masses to determine how much mass the object has.

In a science classroom, you may use a triple beam balance or an electronic balance. The measurement might be written like this:

$$m = 250\ \text{g}$$

This means the mass \,\((m)\) of the object is 250 grams.

Worked Example 1: Deciding if something is matter

Question: Is sunlight matter?

Step 1: Ask whether sunlight has mass.

Step 2: Ask whether sunlight takes up space like a solid, liquid, or gas.

Answer: Sunlight is not matter. It is a form of energy, not something with mass and volume like objects around us.

Worked Example 2: Identifying matter in everyday life

Question: Which of these are matter: orange juice, air, sound, and a notebook?

Step 1: Check each item for mass and volume.

• Orange juice: has mass and takes up space, so it is matter.
• Air: has mass and takes up space, so it is matter.
• Sound: does not have mass and volume in the same way matter does, so it is not matter.
• Notebook: has mass and takes up space, so it is matter.

Answer: Orange juice, air, and a notebook are matter. Sound is not matter.

Worked Example 3: Converting mass units

Question: A bag of rice has a mass of 2 kilograms. How many grams is that?

Step 1: Use the conversion:

$$1\ \text{kg} = 1000\ \text{g}$$

Step 2: Multiply by 2:

$$2\ \text{kg} = 2 \times 1000\ \text{g}$$

$$2\ \text{kg} = 2000\ \text{g}$$

Answer: The bag of rice has a mass of 2000 g.

Worked Example 4: Comparing mass and size

Question: A small metal cube has a mass of 300 g. A larger foam cube has a mass of 50 g. Which one has more matter?

Step 1: Remember that mass tells us the amount of matter.

Step 2: Compare the masses:

$$300\ \text{g} > 50\ \text{g}$$

Answer: The small metal cube has more matter, even though it is smaller in size.

Common mistakes to avoid

• Thinking only visible things are matter: Air is invisible, but it is still matter.
• Thinking bigger means more mass: Size and mass are not always the same.
• Confusing mass with weight: Mass is the amount of matter. Weight depends on gravity.
• Thinking energy is matter: Light, heat, and sound are not matter.

Why matter and mass are important

Understanding matter and mass helps you explain the physical world. It helps you sort things into what is matter and what is not. It also helps you describe objects in a scientific way.

Later, when you study atoms, elements, compounds, and changes in matter, you will keep using these same ideas. Matter is the “stuff” everything is made of, and mass helps us measure how much of that stuff is present.

Quick review

• Matter is anything that has mass and volume.
• Mass is the amount of matter in an object.
• Volume is the amount of space an object takes up.
• Solids, liquids, and gases are all matter.
• Air is matter even though you usually cannot see it.
• Light, sound, and heat are not matter.
• Mass is commonly measured in grams and kilograms.

Conclusion

Matter is all around you. If something has mass and takes up space, it is matter. Mass tells you how much matter is present, and volume tells you how much space it fills.

By learning to recognize matter and measure mass, you build a strong foundation for understanding the rest of science. These ideas help explain everything from a grain of sand to the air in the sky.

Kinetic Molecular Theory is a big name for a simple idea: all matter is made of tiny particles that are always moving.

Matter is anything that takes up space and has mass. The air in a balloon, the water in a cup, and the ice in a freezer are all matter. Even though we may not be able to see the tiny particles, they are there.

The Kinetic Molecular Theory helps us explain why solids, liquids, and gases act differently. It also helps us understand what happens when matter gets warmer or cooler.

The word kinetic means motion, or movement. So this theory is about how tiny particles move.

1. Matter is made of tiny particles

Everything around us is made of very small pieces called particles. They are much too tiny to see without special tools.

These particles can be close together or spread apart. How close they are, and how much they move, helps decide whether matter is a solid, liquid, or gas.

2. The particles are always moving

One of the most important ideas is that particles are never completely still. They are always moving, even in things that look still, like a rock or an ice cube.

In some kinds of matter, particles move only a little. In others, they move more freely and quickly.

• In solids, particles are packed closely together and mostly wiggle in place.
• In liquids, particles are still close together, but they can slide past each other.
• In gases, particles are far apart and move around quickly in many directions.

3. Temperature changes how fast particles move

Temperature tells us how warm or cool something is. In Kinetic Molecular Theory, temperature is connected to how fast the particles are moving.

When matter is heated, its particles usually move faster. When matter is cooled, its particles usually move slower.

We can think of it like this:

$$\text{more heat} \rightarrow \text{faster particle motion}$$

$$\text{less heat} \rightarrow \text{slower particle motion}$$

Faster movement means the particles have more kinetic energy. Slower movement means they have less kinetic energy.

You do not need to memorize a hard definition. Just remember: more kinetic energy means more movement.

4. How particle motion explains the states of matter

The way particles move helps explain the states of matter: solid, liquid, and gas.

Solid: In a solid, particles are very close together. They do not move around freely. They mostly vibrate or wiggle in place. That is why solids keep their shape.

Liquid: In a liquid, particles are still close, but they can move past one another. That is why liquids can be poured and take the shape of their container.

Gas: In a gas, particles are spread far apart and move very fast. That is why gases spread out and fill the space they are in.

5. Heating and cooling can change the state of matter

Because temperature changes particle motion, heating and cooling can change matter from one state to another.

• Melting: A solid gains heat, its particles move faster, and it can become a liquid.
• Freezing: A liquid loses heat, its particles move slower, and it can become a solid.
• Evaporation: A liquid gains heat, some particles move fast enough to become a gas.
• Condensation: A gas loses heat, its particles slow down, and it can become a liquid.

For example, an ice cube melts into water when it gets warmer. Water can turn into water vapor when it is heated. Water vapor can turn back into liquid water when it cools.

6. What we can observe with our eyes

We usually cannot see particles, but we can see clues that show they are moving.

• A drop of food coloring slowly spreads through water.
• The smell of popcorn travels through a room.
• Steam rises from hot soup.
• An ice cube melts on a warm day.

These are signs that tiny particles are moving and spreading out.

Worked Example 1: Ice, water, and steam

Question: How are the particles different in ice, liquid water, and steam?

Step 1: Think about the state of matter.

• Ice is a solid.
• Water is a liquid.
• Steam is a gas.

Step 2: Match each state with particle motion.

• In ice, particles are close together and wiggle in place.
• In water, particles are close together but slide past one another.
• In steam, particles are far apart and move quickly.

Answer: As water goes from ice to liquid to steam, the particles move more and spread farther apart.

Worked Example 2: Why does a balloon get bigger in warm air?

Question: A balloon is brought from a cool room into a warm room. Why might it get a little bigger?

Step 1: The balloon has gas particles inside it.

Step 2: In the warm room, the gas particles get more heat.

Step 3: When they get more heat, they move faster.

Step 4: Faster-moving particles push outward more.

Answer: The balloon may get bigger because the gas particles inside move faster in the warm air.

Worked Example 3: Food coloring in water

Question: A student drops blue food coloring into a cup of water. After a while, the color spreads through the whole cup. Why?

Step 1: Water is made of moving particles.

Step 2: The food coloring is also made of tiny particles.

Step 3: Since particles are always moving, the coloring particles spread out through the water.

Answer: The color spreads because tiny particles in the water and the food coloring are always moving.

Worked Example 4: Which has faster-moving particles?

Question: Which usually has faster-moving particles: hot soup or cold soup?

Step 1: Remember the rule: more heat means faster particle motion.

Step 2: Hot soup has a higher temperature than cold soup.

Answer: Hot soup usually has faster-moving particles.

7. Important ideas to remember

• All matter is made of tiny particles.
• These particles are always moving.
• Temperature affects how fast the particles move.
• Heating usually makes particles move faster.
• Cooling usually makes particles move slower.
• The motion and spacing of particles help explain solids, liquids, and gases.

8. A simple way to picture it

Imagine students in a classroom:

• Solid: Everyone stands very close in one spot and only wiggles a little.
• Liquid: Everyone stays close, but can slowly move around each other.
• Gas: Everyone spreads out and moves quickly all around the room.

This is not exactly what particles do, but it is a helpful way to picture the idea.

Brief Summary

The Kinetic Molecular Theory says that all matter is made of tiny particles that are always moving. The warmer matter gets, the faster its particles move. The cooler matter gets, the slower they move. This helps explain why solids, liquids, and gases behave in different ways.

States of Matter describes the different forms that matter can take. Matter is anything that has mass and takes up space. Everything around you is made of matter, including the air you breathe, the water you drink, and the chair you sit on.

The main states of matter you will study are solid, liquid, gas, and plasma. These states are different because the particles in each state move differently and are arranged in different ways.

Even though we cannot usually see particles with our eyes, it helps to imagine that all matter is made of tiny pieces. The way these particles are spaced and how much they move helps explain why each state of matter has its own properties.

Why states of matter matter: If you understand how particles behave, you can explain why ice keeps its shape, why juice can be poured, why steam spreads out, and why the Sun glows as plasma.

1. Solids

A solid has a definite shape and a definite volume. This means a solid keeps its own shape and takes up a set amount of space unless something changes it.

In a solid, particles are packed closely together. They do not move around freely, but they do vibrate in place. Because the particles are so close together, solids are usually hard to squeeze or compress.

• Definite shape
• Definite volume
• Particles packed tightly
• Particles vibrate in place
• Not easy to compress

Examples of solids include:

• Ice
• A brick
• A metal spoon
• A book

2. Liquids

A liquid has a definite volume but not a definite shape. A liquid keeps the same amount of matter, but it changes shape to fit its container.

In a liquid, particles are still close together, but they can slide past one another. This movement lets liquids flow. Liquids are also difficult to compress because their particles are still fairly close together.

• No definite shape
• Definite volume
• Particles close together
• Particles slide past one another
• Can flow

Examples of liquids include:

• Water
• Milk
• Juice
• Cooking oil

3. Gases

A gas has no definite shape and no definite volume. A gas spreads out to fill whatever container it is in.

In a gas, particles are much farther apart than in solids or liquids. They move quickly in many directions. Because there is so much space between particles, gases can be compressed more easily than solids and liquids.

• No definite shape
• No definite volume
• Particles far apart
• Particles move quickly and freely
• Can be compressed

Examples of gases include:

• Air
• Oxygen
• Helium in a balloon
• Water vapor

4. Plasma

Plasma is sometimes called the fourth state of matter. Plasma is like a gas in some ways, but it has so much energy that its particles are electrically charged.

Plasma is found in stars, including the Sun. It is also found in lightning and in some neon signs. Plasma is less common in everyday life than solids, liquids, and gases, but it is very important in space and in certain kinds of technology.

• No definite shape
• No definite volume
• Very high energy
• Particles are electrically charged

Examples of plasma include:

• The Sun
• Other stars
• Lightning
• Neon lights

How particle motion explains the states

The amount of energy particles have affects how fast they move. When particles have less energy, they move less. When they have more energy, they move more.

In general:

• Particles in solids have the least movement.
• Particles in liquids move more than in solids.
• Particles in gases move much more freely.
• Particles in plasma have very high energy.

This is why heating and cooling can change the state of matter. Adding energy usually makes particles move faster. Removing energy usually makes particles move more slowly.

Changes of state

Matter can change from one state to another when energy is added or removed. These changes are called changes of state or phase changes.

1. Melting: solid to liquid
2. Freezing: liquid to solid
3. Evaporation: liquid to gas
4. Boiling: liquid to gas throughout the liquid
5. Condensation: gas to liquid
6. Sublimation: solid to gas
7. Deposition: gas to solid

For example, when ice is heated, it melts into liquid water. If the water is heated more, it can evaporate or boil and become water vapor, which is a gas. If water vapor cools, it can condense back into liquid water.

A simple way to show these changes is:

$$ \text{solid} \rightarrow \text{liquid} \rightarrow \text{gas} $$

and when cooling:

$$ \text{gas} \rightarrow \text{liquid} \rightarrow \text{solid} $$

Heating and cooling example with water

Water is a great example because it can be seen in all three common states on Earth.

• Ice is solid water.
• Liquid water is the form you drink.
• Water vapor is gaseous water in the air.

At standard pressure, water freezes at \(0^\circ\text{C}\) and boils at \(100^\circ\text{C}\). These temperatures help us describe when water changes state.

If the temperature rises from \(0^\circ\text{C}\) to \(100^\circ\text{C}\), the temperature change is:

$$ 100 - 0 = 100^\circ\text{C} $$

This shows a temperature increase of \(100^\circ\text{C}\).

Comparing the four states

Worked Example 1: Identifying a state of matter

Question: A material keeps the same volume but changes shape to fit its cup. What state of matter is it?

Step 1: Look for clues. It has a definite volume.

Step 2: It does not have a definite shape because it fits the cup.

Answer: It is a liquid.

Worked Example 2: Describing particle behavior

Question: Which state of matter has particles that are far apart and move freely?

Step 1: Recall the particle model.

• Solid: tightly packed, vibrate
• Liquid: close together, slide
• Gas: far apart, move freely

Answer: The state is gas.

Worked Example 3: Change of state

Question: Ice cubes are left on a table and become liquid water. What change of state happened?

Step 1: Ice is a solid.

Step 2: Liquid water is a liquid.

Step 3: A change from solid to liquid is called melting.

Answer: The ice melted.

Worked Example 4: Using temperature difference

Question: Water is heated from \(20^\circ\text{C}\) to \(75^\circ\text{C}\). By how many degrees did the temperature increase?

Step 1: Subtract the starting temperature from the ending temperature.

$$ 75 - 20 = 55 $$

Answer: The temperature increased by \(55^\circ\text{C}\).

Common mistakes to avoid

• Mistake: Thinking gases have no mass.
  Correction: Gases are matter, so they do have mass.
• Mistake: Thinking particles in solids do not move at all.
  Correction: They vibrate in place.
• Mistake: Thinking liquids have no volume.
  Correction: Liquids have definite volume but not definite shape.
• Mistake: Confusing evaporation and condensation.
  Correction: Evaporation is liquid to gas; condensation is gas to liquid.

Real-life connections

You see states of matter every day. Ice in a freezer is a solid. Rain is liquid water. Water vapor in the air is a gas. Lightning is an example of plasma.

Understanding states of matter also helps in cooking, weather, and technology. Boiling water, freezing food, and the formation of clouds all involve changes of state.

Brief Summary

Matter exists mainly as solids, liquids, gases, and plasmas. These states are different because of how their particles are arranged and how they move. Solids keep shape and volume, liquids keep volume but change shape, gases change both shape and volume, and plasma is a high-energy state with charged particles.

When matter gains or loses energy, it can change state through melting, freezing, evaporation, condensation, sublimation, or deposition. By using the particle model, you can explain the properties of each state and the changes between them.

Kinetic Molecular Theory explains matter by looking at the tiny particles it is made of. Even though we cannot usually see these particles, their motion helps explain why solids keep their shape, why liquids flow, and why gases spread out to fill a container.

The word kinetic means motion. The word molecular refers to the tiny particles in matter, such as atoms or molecules. So, Kinetic Molecular Theory is the idea that all matter is made of tiny particles that are always moving.

This theory helps connect what happens on a tiny scale to what we see on a large scale. For example, when water boils, it looks like bubbling liquid. On the particle level, the water particles are moving faster and spreading farther apart.

Main Idea of Kinetic Molecular Theory

Kinetic Molecular Theory has several important points:

• All matter is made of tiny particles.
• These particles are always moving.
• When particles gain energy, they move faster.
• When particles lose energy, they move slower.
• Particles collide with each other and with the walls of their container.
• There is space between particles, especially in gases.

These ideas explain many everyday observations, such as melting ice, evaporation, and why a balloon can expand.

Particles in the Three States of Matter

The way particles move and how close they are to each other help determine whether matter is a solid, liquid, or gas.

1. Solids

• Particles are packed very closely together.
• They do not move from place to place easily.
• They mostly vibrate in place.
• This is why solids have a definite shape and definite volume.

An ice cube keeps its shape because its particles are tightly packed and only vibrate.

2. Liquids

• Particles are still close together, but not as tightly packed as in a solid.
• They can slide past one another.
• This is why liquids have a definite volume but take the shape of their container.

Water in a glass changes shape if poured into a bowl, but the amount of water stays the same.

3. Gases

• Particles are far apart compared with solids and liquids.
• They move quickly in many directions.
• They spread out to fill any container.
• This is why gases do not have a definite shape or definite volume.

Air fills a classroom because gas particles move freely and spread out.

Temperature and Particle Motion

Temperature is related to how fast particles are moving. When temperature increases, particles move faster. When temperature decreases, particles move slower.

This does not mean every particle moves at exactly the same speed, but in general, warmer matter has faster-moving particles than cooler matter.

If energy is added to matter, the particles gain motion. If energy is removed, the particles lose motion. This is why heating and cooling can change the state of matter.

• Heating a solid can make it melt into a liquid.
• Heating a liquid can make it become a gas.
• Cooling a gas can make it condense into a liquid.
• Cooling a liquid can make it freeze into a solid.

Collisions and Pressure

Gas particles are always moving and colliding. They bump into each other and into the walls of their container. These collisions create pressure.

For example, the air inside a basketball pushes outward because tiny gas particles are hitting the inside walls of the ball.

If gas particles move faster, they hit the walls harder and more often. This can increase pressure.

Why Gases Can Be Compressed

Gases can be squeezed into a smaller space because there is a lot of empty space between their particles. Solids and liquids cannot be compressed as easily because their particles are already close together.

This is why air can be pumped into a tire. The gas particles are forced closer together.

How Kinetic Molecular Theory Explains Changes of State

Changes of state happen when particle motion changes enough to change how strongly particles stay together.

1. Melting: A solid gains energy. Its particles vibrate faster until they can move past one another.
2. Freezing: A liquid loses energy. Its particles slow down and lock into place.
3. Evaporation/Boiling: A liquid gains energy. Some or all particles move fast enough to become a gas.
4. Condensation: A gas loses energy. Its particles slow down and come closer together as a liquid.

These changes do not create new matter. The same particles are still present. What changes is their motion and arrangement.

Real-Life Examples

• Perfume spreading through a room: Gas particles move freely and mix with air.
• An ice cube melting: Particles gain energy and move more freely.
• A balloon shrinking in the cold: Gas particles slow down, so they do not push outward as much.
• Boiling water: Water particles move fast enough to escape as gas.

Worked Example 1: Comparing Particle Motion

Question: Which has faster-moving particles: a cup of hot tea or a cup of iced tea?

Step 1: Remember that higher temperature means faster particle motion.

Step 2: Hot tea has a higher temperature than iced tea.

Answer: The hot tea has faster-moving particles.

Worked Example 2: Explaining a State of Matter

Question: Why does a gas fill the entire container it is in?

Step 1: Think about how gas particles are arranged.

Step 2: Gas particles are far apart and move quickly in all directions.

Step 3: Because they move freely, they spread out as much as possible.

Answer: A gas fills its container because its particles are far apart and move freely in all directions.

Worked Example 3: Heating a Balloon

Question: A balloon is left in a warm room. What happens to the gas particles inside, and what might happen to the balloon?

Step 1: Warmer temperature gives particles more energy.

Step 2: The gas particles move faster.

Step 3: Faster particles hit the inside of the balloon more often and with more force.

Answer: The particles move faster, and the balloon may expand because the particles push outward more strongly.

Worked Example 4: Cooling Water Vapor

Question: When water vapor on a bathroom mirror turns into liquid water, what is happening to the particles?

Step 1: Water vapor is a gas, so its particles are moving quickly and are far apart.

Step 2: The cooler mirror removes energy from the particles.

Step 3: The particles slow down and come closer together.

Answer: The particles lose energy, slow down, and condense into liquid water.

Simple Math Connection

We can think about pressure in a simple way. If the number of collisions increases, pressure increases. If collisions decrease, pressure decreases.

This can be shown as a basic relationship:

And when temperature rises for a gas in a closed container, particle motion usually increases:

Common Mistakes to Avoid

• Mistake: Particles in solids do not move.
  Correction: They do move, but mainly by vibrating in place.
• Mistake: Heat and temperature are exactly the same.
  Correction: Temperature tells how fast particles are moving on average.
• Mistake: When matter changes state, it becomes a different substance.
  Correction: It is still the same substance; only the particle motion and spacing change.
• Mistake: Gases have no particles because we cannot see them.
  Correction: Gases are made of particles that are too small to see easily.

Why This Theory Matters

Kinetic Molecular Theory is important because it helps explain many things in science and daily life. It shows why materials behave the way they do when heated, cooled, squeezed, or left in open space.

It also helps us understand weather, cooking, breathing, and even why smells travel through the air.

Brief Summary

Kinetic Molecular Theory says that all matter is made of tiny particles that are always moving. The speed of particle motion depends on temperature. The arrangement and movement of particles explain the properties of solids, liquids, and gases, as well as changes of state and pressure in gases.

Phase Transitions are changes in the state of matter. Matter can exist as a solid, liquid, or gas. When matter changes from one state to another, it goes through a phase transition.

These changes happen because of energy, usually in the form of heat. When matter gains heat, its particles usually move faster. When matter loses heat, its particles usually move slower.

Understanding phase transitions helps explain many everyday events, like ice melting, water boiling, fog forming, and frost appearing on grass.

States of Matter Review

• Solid: Particles are packed closely together and mostly vibrate in place. Solids keep their shape.
• Liquid: Particles are still close together, but they can slide past one another. Liquids take the shape of their container.
• Gas: Particles are far apart and move freely. Gases spread out to fill their container.

Why Phase Transitions Happen

All matter is made of tiny particles. These particles are always moving. The amount of heat energy in a substance affects how much the particles move.

When a substance gains heat energy, particles move more and may spread farther apart. This can cause a solid to melt or a liquid to evaporate.

When a substance loses heat energy, particles move less and may come closer together. This can cause a gas to condense or a liquid to freeze.

The Six Main Phase Transitions

1. Melting – changing from a solid to a liquid
2. Freezing – changing from a liquid to a solid
3. Vaporization – changing from a liquid to a gas
4. Condensation – changing from a gas to a liquid
5. Sublimation – changing from a solid directly to a gas
6. Deposition – changing from a gas directly to a solid

1. Melting

Melting happens when a solid gains enough heat energy to become a liquid. The particles in the solid vibrate faster and begin to break out of their fixed positions.

A common example is ice melting into liquid water. For pure water, melting happens at about \(0^\circ\text{C}\).

2. Freezing

Freezing is the opposite of melting. It happens when a liquid loses heat energy and becomes a solid. The particles slow down and lock into place.

Water freezes into ice at about \(0^\circ\text{C}\). Even though melting and freezing are opposites, they happen at the same temperature for pure water.

3. Vaporization

Vaporization is when a liquid changes into a gas. There are two common ways this happens:

• Evaporation: a slower change that happens at the surface of a liquid
• Boiling: a faster change that happens throughout the liquid

When water boils, it changes into water vapor, which is a gas. Pure water boils at about \(100^\circ\text{C}\) at normal air pressure.

4. Condensation

Condensation happens when a gas loses heat energy and changes into a liquid. The particles slow down and move closer together.

You can see condensation when water droplets form on the outside of a cold glass. Water vapor in the air cools and turns into liquid water.

5. Sublimation

Sublimation is when a solid changes directly into a gas without becoming a liquid first.

A well-known example is dry ice. Dry ice is solid carbon dioxide, and it changes directly into carbon dioxide gas.

6. Deposition

Deposition is the opposite of sublimation. It happens when a gas changes directly into a solid without becoming a liquid first.

Frost is a good example. On a cold morning, water vapor in the air can turn directly into solid ice on windows or grass.

Heating and Cooling Changes

Some phase transitions happen when matter is heated, and others happen when matter is cooled.

• Heating changes: melting, vaporization, sublimation
• Cooling changes: freezing, condensation, deposition

Particle Model of Phase Changes

It helps to imagine matter at the particle level:

• In a solid, particles are tightly packed and mostly vibrate.
• In a liquid, particles are close but can move around each other.
• In a gas, particles move quickly and spread far apart.

During a phase transition, the particles themselves do not change into a different substance. The substance stays the same, but the arrangement and motion of the particles change.

For example, ice, liquid water, and water vapor are all made of water particles. They are the same substance in different states.

Important Idea: A Change of State Is Usually a Physical Change

Phase transitions are usually physical changes, not chemical changes. That means the substance is still the same substance after the change.

When ice melts, it is still water. When liquid water evaporates, it is still water. No new substance is made.

Temperature and Phase Changes

When a substance is being heated or cooled, its temperature often changes. But during the actual phase transition, the temperature can stay the same for a while as energy is used to change the state.

For example, ice at \(0^\circ\text{C}\) can keep melting for a while before the temperature of the water rises above \(0^\circ\text{C}\).

You do not need to memorize difficult formulas for this. Just remember this simple idea:

$$\text{Heat added} \rightarrow \text{particles move more} \rightarrow \text{matter may change state}$$

$$\text{Heat removed} \rightarrow \text{particles move less} \rightarrow \text{matter may change state}$$

Phase Transition Chart

• Solid \(\rightarrow\) Liquid: Melting
• Liquid \(\rightarrow\) Solid: Freezing
• Liquid \(\rightarrow\) Gas: Vaporization
• Gas \(\rightarrow\) Liquid: Condensation
• Solid \(\rightarrow\) Gas: Sublimation
• Gas \(\rightarrow\) Solid: Deposition

Worked Example 1: Ice Cube on a Plate

Question: An ice cube is left on a kitchen plate. After some time, it becomes liquid water. What phase transition happened?

Step 1: Identify the starting state. The ice cube is a solid.

Step 2: Identify the ending state. The liquid water is a liquid.

Step 3: Match the change. Solid to liquid is melting.

Answer: The ice cube went through melting.

Worked Example 2: Water Drops on a Cold Soda Can

Question: A cold soda can is taken outside on a warm day. Soon, water drops appear on the outside of the can. Where did the liquid water come from, and what phase transition happened?

Step 1: Notice that the water did not leak through the can. It came from water vapor in the air.

Step 2: The air near the can cooled down.

Step 3: Water vapor, which is a gas, changed into liquid water.

Answer: The phase transition is condensation.

Worked Example 3: Frost on the Ground

Question: On a very cold morning, frost appears on grass. What phase transition caused this?

Step 1: Frost is solid ice.

Step 2: The water started as water vapor in the air, which is a gas.

Step 3: Gas changed directly to solid.

Answer: The phase transition is deposition.

Worked Example 4: Identifying More Than One Change

Question: A pot of water is placed in a freezer. First the water becomes ice. Later, the ice is taken out and heated until it becomes liquid water again, and then more heat turns some of it into steam. Name the phase transitions in order.

Step 1: Liquid water to solid ice is freezing.

Step 2: Solid ice to liquid water is melting.

Step 3: Liquid water to steam, a gas, is vaporization.

Answer: The transitions are freezing \(\rightarrow\) melting \(\rightarrow\) vaporization.

Common Mistakes to Avoid

• Mixing up evaporation and condensation: Evaporation is liquid to gas. Condensation is gas to liquid.
• Thinking matter disappears: When a puddle evaporates, the water has not disappeared. It changed into water vapor in the air.
• Confusing sublimation and deposition: Sublimation is solid to gas. Deposition is gas to solid.
• Thinking a new substance forms: During a phase transition, the substance usually stays the same.

Real-Life Examples of Phase Transitions

• Ice cream melting on a hot day
• Water freezing into ice cubes in a tray
• A wet shirt drying by evaporation
• Clouds forming when water vapor condenses
• Dry ice turning into gas by sublimation
• Frost forming by deposition

Quick Review

• Phase transitions are changes between solid, liquid, and gas.
• They happen because matter gains or loses heat energy.
• Heating changes: melting, vaporization, sublimation.
• Cooling changes: freezing, condensation, deposition.
• These are usually physical changes, so the substance stays the same.

Summary

Phase transitions explain how matter changes from one state to another. When heat energy is added, particles move faster and matter may melt, vaporize, or sublimate. When heat energy is removed, particles move slower and matter may freeze, condense, or deposit.

If you remember the starting state, the ending state, and whether heat was added or removed, you can figure out the name of the phase transition.

Heating and Cooling Curves help us understand what happens to a substance when it gains or loses heat. These graphs show how temperature changes over time as matter is heated or cooled.

They are especially useful because they show not only when a substance gets warmer or cooler, but also when it changes state. A substance can be a solid, liquid, or gas. During a state change, the temperature may stay the same even though heat is still being added or removed.

In this lesson, you will learn how to read heating and cooling curves, identify where phase changes happen, and explain why some parts of the graph are flat.

1. What is a heating curve?

A heating curve is a graph that shows how the temperature of a substance changes as heat is added.

Usually, the x-axis shows time or heat added, and the y-axis shows temperature.

A heating curve often has both slanted sections and flat sections:

• Slanted sections: the temperature is changing.
• Flat sections: the temperature stays the same while the substance changes state.

2. What is a cooling curve?

A cooling curve is a graph that shows how the temperature changes as heat is removed.

It has the same basic idea as a heating curve, but the temperature goes down instead of up. Just like in a heating curve, flat sections show that a phase change is happening.

3. The three common states of matter

• Solid: particles are packed closely together and mostly vibrate in place.
• Liquid: particles are still close together, but they can move past one another.
• Gas: particles are far apart and move freely.

When a substance is heated, it may go from solid to liquid to gas. When it is cooled, it may go from gas to liquid to solid.

4. Why do flat parts appear on the graph?

The flat parts are one of the most important features of heating and cooling curves.

During a phase change, the energy being added or removed is not used to change the temperature. Instead, it is used to change the arrangement and movement of the particles.

For example:

• When a solid melts into a liquid, added heat is used to loosen the particles from their fixed positions.
• When a liquid boils into a gas, added heat is used to separate the particles even more.

This energy involved in a phase change is called latent heat. For 7th Grade, you can think of latent heat as hidden energy used for changing state, not changing temperature.

5. Parts of a typical heating curve

Imagine heating an ice cube. A typical heating curve may have five parts:

1. Solid warming: the solid gets hotter, so the temperature rises.
2. Melting: the temperature stays constant while the solid changes to liquid.
3. Liquid warming: the liquid gets hotter, so the temperature rises again.
4. Boiling: the temperature stays constant while the liquid changes to gas.
5. Gas warming: the gas gets hotter, so the temperature rises again.

If this were water, melting happens at \(0^\circ C\) and boiling happens at \(100^\circ C\) under normal conditions.

6. Parts of a typical cooling curve

Now imagine water vapor cooling down. A cooling curve may also have five parts:

1. Gas cooling: the gas loses heat and temperature drops.
2. Condensing: the temperature stays constant while the gas changes to liquid.
3. Liquid cooling: the liquid loses heat and temperature drops more.
4. Freezing: the temperature stays constant while the liquid changes to solid.
5. Solid cooling: the solid keeps losing heat and gets colder.

7. Slanted lines vs. flat lines

• Slanted line upward: the substance is heating up and temperature is increasing.
• Slanted line downward: the substance is cooling down and temperature is decreasing.
• Flat line during heating: the substance is melting or boiling.
• Flat line during cooling: the substance is condensing or freezing.

A quick rule to remember is:

If the line is slanted, temperature is changing. If the line is flat, state is changing.

8. How particle motion explains the graph

Temperature is related to how fast particles are moving. When temperature rises, particles move faster. When temperature falls, particles move slower.

But during a phase change, the energy goes into changing how particles are arranged instead of making them move faster or slower. That is why the temperature stays the same for a while.

9. Reading a heating or cooling curve step by step

When you look at a graph, ask yourself these questions:

1. Is the graph showing heating or cooling?
2. Where are the slanted sections?
3. Where are the flat sections?
4. What state of matter is the substance in during each section?
5. At the flat sections, what phase change is happening?

This method helps you interpret almost any heating or cooling curve.

10. Worked Example 1: Identifying parts of a heating curve

A graph shows these sections as a substance is heated:

• Section A: temperature rises from \(-10^\circ C\) to \(0^\circ C\)
• Section B: temperature stays at \(0^\circ C\)
• Section C: temperature rises from \(0^\circ C\) to \(100^\circ C\)
• Section D: temperature stays at \(100^\circ C\)
• Section E: temperature rises above \(100^\circ C\)

Question: What is happening in each section?

Step 1: Rising temperature means the substance is warming within one state of matter.

Step 2: Flat temperature means a phase change is happening.

Answer:

• Section A: solid warming
• Section B: melting
• Section C: liquid warming
• Section D: boiling
• Section E: gas warming

This is a classic heating curve for water.

11. Worked Example 2: Identifying parts of a cooling curve

A substance starts as a gas. On the graph, the temperature drops, then stays flat, then drops again, then stays flat again.

Question: What changes are happening?

Step 1: A downward slanted line means the substance is cooling in one state.

Step 2: The first flat part means gas is changing into liquid. This is condensation.

Step 3: The next downward slanted line means the liquid is cooling.

Step 4: The second flat part means liquid is changing into solid. This is freezing.

Answer: The graph shows gas cooling, condensing, liquid cooling, and freezing.

12. Worked Example 3: Why doesn’t the temperature rise?

A student is heating a solid. The thermometer reaches \(50^\circ C\) and then stays at \(50^\circ C\) for several minutes, even though the heater is still on.

Question: Why does the temperature stay the same?

Answer: The substance is going through a phase change at \(50^\circ C\). The added energy is being used as latent heat to change the substance from one state to another, not to increase temperature.

If the substance started as a solid, it is most likely melting. If it started as a liquid, it may be boiling.

13. Worked Example 4: Comparing two sections

On a heating curve, Section X is a slanted line and Section Y is a flat line.

Question: In which section are the particles gaining energy without a temperature increase?

Answer: Section Y, the flat line.

During the flat section, the particles are still gaining energy, but that energy is used to change state. The temperature does not rise until the phase change is complete.

14. Common mistakes to avoid

• Mistake 1: Thinking a flat line means nothing is happening. Actually, a phase change is happening.
• Mistake 2: Thinking heat always makes temperature go up. During melting or boiling, heat can be added while temperature stays constant.
• Mistake 3: Forgetting that cooling curves also have flat sections. Condensation and freezing also happen at constant temperature.
• Mistake 4: Mixing up temperature change and state change. Slanted parts show temperature change; flat parts show state change.

15. Key words to know

• Heating curve: a graph showing temperature changes as heat is added.
• Cooling curve: a graph showing temperature changes as heat is removed.
• Phase change: a change from one state of matter to another.
• Melting: solid to liquid.
• Boiling: liquid to gas.
• Condensation: gas to liquid.
• Freezing: liquid to solid.
• Latent heat: energy used to change state without changing temperature.

16. A simple way to remember heating and cooling curves

• Slant = temperature changes
• Flat = state changes
• Heating: solid  liquid  gas
• Cooling: gas  liquid  solid

You can also remember that melting and boiling happen during heating, while condensation and freezing happen during cooling.

17. Brief summary

Heating and cooling curves show how a substance changes temperature and state as heat is added or removed. Slanted parts of the graph show temperature changing within a single state of matter. Flat parts show phase changes, where latent heat is used and the temperature stays constant. By reading these sections carefully, you can tell whether a substance is a solid, liquid, or gas and when it is melting, boiling, condensing, or freezing.

Extensive vs. Intensive Properties

When scientists describe matter, they often talk about its properties. A property is a characteristic you can observe or measure. For example, you might describe an object by its mass, volume, color, or temperature.

Some properties change when the amount of matter changes. Other properties stay the same even if the amount changes. This is the big idea behind extensive and intensive properties.

Learning the difference helps you answer questions like these:

• Does a bigger sample have different properties than a smaller sample?
• Can this property help identify a substance?
• Does cutting something in half change the property?

1. What is an extensive property?

An extensive property is a property that depends on the amount of matter present.

If you have more of the substance, the value of the property changes. If you have less of the substance, the value changes too.

Common examples of extensive properties are:

• Mass — how much matter is in an object
• Volume — how much space the object takes up
• Length — how long something is
• Amount — how much of the material you have

For example, a full bottle of water has more mass than a half-full bottle of water. It also has more volume. That means mass and volume are extensive properties.

2. What is an intensive property?

An intensive property is a property that does not depend on the amount of matter.

Even if you cut the material into smaller pieces, this property stays the same for each piece, as long as the substance itself stays the same.

Common examples of intensive properties are:

• Density — how tightly packed the matter is
• Color
• Temperature
• Melting point
• Boiling point
• Hardness

For example, a drop of pure water and a bucket of pure water have the same boiling point under the same conditions. That means boiling point is an intensive property.

3. A simple way to tell the difference

Ask yourself this question:

If I take half of the sample, will this property also be cut in half or change because the amount changed?

• If yes, it is probably an extensive property.
• If no, it is probably an intensive property.

For example:

• If you cut a clay block in half, the mass of each half is less than the whole. So mass is extensive.
• If you cut the same clay block in half, the color stays the same. So color is intensive.

4. Why density is intensive

Density can be tricky because it uses mass and volume, and both of those are extensive. But density itself is intensive.

Density is found using this formula:

$$\text{density} = \frac{\text{mass}}{\text{volume}}$$

If you cut a sample in half, both the mass and the volume are reduced by the same amount. Because they change together, the density stays the same.

That is why density can help identify a substance, but mass and volume alone usually cannot.

5. Comparing extensive and intensive properties

• Extensive properties tell you about how much matter there is.
• Intensive properties tell you about what kind of matter it is.

This does not mean intensive properties always identify a substance by themselves, but they are often more helpful for doing that.

Here is a quick comparison:

• Mass: changes with amount → extensive
• Volume: changes with amount → extensive
• Temperature: stays the same for the sample → intensive
• Density: stays the same for the substance → intensive
• Color: stays the same for the substance → intensive

6. Worked Examples

Example 1: Classifying simple properties

A student has a rock. She writes down these properties: mass, color, and volume. Which are extensive and which are intensive?

Step 1: Ask whether the property changes when the amount changes.

• Mass: A larger rock has more mass than a smaller piece. So mass depends on amount.
• Volume: A larger rock takes up more space. So volume depends on amount.
• Color: Breaking the rock into pieces does not usually change its color. So color does not depend on amount.

Answer:

• Mass → extensive
• Volume → extensive
• Color → intensive

Example 2: Cutting a sample in half

You have a metal bar with a mass of 200 g and a length of 20 cm. You cut it into two equal pieces.

What happens to the mass and length of each piece? Are these properties extensive or intensive?

Step 1: Divide the sample into two equal parts.

Each piece has:

• Mass: \(200 \div 2 = 100\) g
• Length: \(20 \div 2 = 10\) cm

Step 2: Notice that both values changed because the amount of matter changed.

Answer: Mass and length are both extensive properties.

Example 3: Understanding density

A block of wood has a mass of 60 g and a volume of 100 cm³. Its density is:

$$\text{density} = \frac{60}{100} = 0.6\ \text{g/cm}^3$$

Now the block is cut into two equal pieces.

Each piece has:

• Mass: \(30\) g
• Volume: \(50\) cm³

Find the density of one piece:

$$\text{density} = \frac{30}{50} = 0.6\ \text{g/cm}^3$$

The density stayed the same, even though the amount of wood changed.

Answer: Density is an intensive property.

Example 4: Which property helps identify the substance?

Two clear liquids are in different cups. One cup holds a small amount. The other cup holds a large amount. A student wants to know whether the liquids might be the same substance.

He measures these properties:

• Cup A: mass = 50 g, boiling point = 100°C
• Cup B: mass = 200 g, boiling point = 100°C

Step 1: Compare the mass.

The masses are different, but that may only mean the amounts are different. Mass is extensive.

Step 2: Compare the boiling point.

The boiling point is the same in both samples. Boiling point is intensive.

Answer: The matching boiling point is more useful for telling whether the liquids could be the same substance.

7. Common mistakes to avoid

• Mistake 1: Thinking that all measured properties are extensive. Some measured properties, like temperature and density, are intensive.
• Mistake 2: Thinking bigger samples must have different intensive properties. A larger sample of the same pure substance usually has the same density, melting point, and boiling point.
• Mistake 3: Forgetting to ask whether the property depends on the amount of matter. This question is the key to sorting properties correctly.

8. Practice check: Is it extensive or intensive?

• Mass → Extensive
• Volume → Extensive
• Density → Intensive
• Color → Intensive
• Boiling point → Intensive
• Length → Extensive
• Temperature → Intensive

9. Summary

Extensive properties depend on how much matter you have. Examples include mass, volume, and length.

Intensive properties do not depend on the amount of matter. Examples include density, color, temperature, melting point, and boiling point.

If you are unsure, imagine dividing the sample into smaller pieces. If the property changes because the sample is smaller, it is extensive. If it stays the same, it is intensive.

Density and Buoyancy

Have you ever wondered why a huge ship made of metal can float, but a small metal coin sinks? Or why some objects float in water but sink in oil? The answers come from two important science ideas: density and buoyancy.

In this lesson, you will learn what density means, how to calculate it, and how buoyancy helps explain why objects float, sink, or stay suspended in a liquid. These ideas help us understand boats, submarines, balloons, and even why people float more easily in salt water.

1. What is density?

Density tells us how much matter is packed into a certain amount of space. In science, matter is measured by mass, and the amount of space an object takes up is called its volume.

An object with a lot of mass in a small volume has high density. An object with less mass in the same volume has low density.

The formula for density is:

Or in symbols:

Where:

• d = density
• m = mass
• V = volume

Common units for density are grams per cubic centimeter, written as \(g/cm^3\), or grams per milliliter, written as \(g/mL\). For liquids, \(1\,mL = 1\,cm^3\), so these units are closely related.

2. Understanding density with everyday examples

Imagine two boxes that are the same size. One is filled with feathers, and the other is filled with rocks. The box of rocks has much more mass in the same amount of space, so it has a higher density.

Now imagine two blocks with the same mass. One is tiny and one is large. The tiny one has that mass packed into less space, so it has greater density.

This is why density is not just about how heavy something is. A large object can be less dense than a small object if its mass is spread out through more volume.

3. How to calculate density

To calculate density, divide the mass by the volume.

Worked Example 1: Finding density

A block has a mass of 20 g and a volume of 5 mL. What is its density?

Step 1: Write the formula.

Step 2: Substitute the values.

Step 3: Solve.

Answer: The density of the block is 4 g/mL.

4. Using the density formula in different ways

You can rearrange the density formula to find mass or volume if you know the other two values.

• To find mass: \(m = d \times V\)
• To find volume: \(V = \frac{m}{d}\)

Worked Example 2: Finding mass

A liquid has a density of \(2\,g/mL\) and a volume of \(6\,mL\). What is its mass?

Step 1: Use the formula for mass.

Step 2: Substitute the values.

Step 3: Solve.

Answer: The mass is 12 g.

5. Density and floating or sinking

Density helps us predict whether an object will float or sink in a liquid.

• If an object is less dense than the liquid, it will float.
• If an object is more dense than the liquid, it will sink.
• If an object has about the same density as the liquid, it may stay suspended, meaning it neither sinks quickly nor floats strongly.

Water has a density of about \(1\,g/mL\). This is a very useful comparison point.

• Objects with density less than \(1\,g/mL\) usually float in water.
• Objects with density greater than \(1\,g/mL\) usually sink in water.

For example, many kinds of wood float because their density is less than water. A rock usually sinks because its density is greater than water.

Worked Example 3: Predict float or sink

An object has a mass of 18 g and a volume of 24 mL. Will it float or sink in water?

Step 1: Find the density.

Step 2: Compare to water, which is \(1\,g/mL\).

Since \(0.75\,g/mL < 1\,g/mL\), the object is less dense than water.

Answer: The object will float in water.

6. What is buoyancy?

Buoyancy is the upward force that a liquid pushes on an object placed in it. This upward push is called the buoyant force.

When you push a ball underwater, the water pushes up on it. That upward push is buoyancy. If the buoyant force is strong enough, the object floats.

Buoyancy happens because liquids push on objects from all sides. The push from below is stronger than the push from above, so the object gets an overall upward force.

7. Archimedes' principle

A scientist named Archimedes explained buoyancy with a very important idea called Archimedes' principle.

Archimedes' principle: The buoyant force on an object is equal to the weight of the liquid the object pushes aside, or displaces.

This means:

• The more liquid an object displaces, the greater the buoyant force.
• If the buoyant force is greater than or equal to the object's weight, the object can float.
• If the buoyant force is less than the object's weight, the object sinks.

8. The meaning of displacement

When an object is placed in water, it pushes some water out of the way. This is called displacement.

If you put a rock into a full container of water, some water spills out. The spilled water is the displaced water. The amount of displaced water helps determine the buoyant force.

A larger object usually displaces more liquid than a smaller one. That can increase buoyant force.

9. Why ships float

A ship may be made of steel, which is denser than water. But the ship is hollow and filled with air, so its overall density is less than water.

The ship also has a wide shape that displaces a large amount of water. This creates a large buoyant force. As long as the buoyant force balances the ship's weight, the ship floats.

This is why shape matters. A solid steel ball sinks, but a steel ship can float.

10. Floating, sinking, and suspended objects

An object in a liquid can behave in three main ways:

• Float: The buoyant force balances the object's weight before it is fully underwater.
• Sink: Even when fully underwater, the buoyant force is not enough to support the object's weight.
• Stay suspended: The object's density is very close to the liquid's density, so it remains in the middle.

Fish can change how deep they are in water by changing their buoyancy. Submarines do something similar by taking water in or pushing water out of special tanks.

11. Different liquids have different densities

An object may float in one liquid and sink in another because liquids have different densities.

Salt water is denser than fresh water. Because of this, people float more easily in salt water. The salt water can provide a greater buoyant force.

Oil is usually less dense than water, so oil often floats on top of water. If you place an object into both liquids, it might sink in oil but float in water depending on its density.

Worked Example 4: Comparing an object to different liquids

An object has a density of \(0.92\,g/mL\). Predict what happens in these liquids:

• Water: \(1.00\,g/mL\)
• Oil: \(0.90\,g/mL\)

Step 1: Compare the object's density to water.

\(0.92 < 1.00\), so the object is less dense than water.

In water: It will float.

Step 2: Compare the object's density to oil.

\(0.92 > 0.90\), so the object is more dense than oil.

In oil: It will sink.

Answer: The object floats in water but sinks in oil.

12. Measuring density in the lab

To find the density of a solid object, you measure its mass and volume.

• Mass can be measured with a balance.
• Volume of a regular solid can be found by measuring length, width, and height.
• Volume of an irregular solid can be found using water displacement.

For water displacement, place water in a graduated cylinder and record the starting volume. Then place the object in the water and record the new volume. The difference is the object's volume.

For example, if the water rises from \(30\,mL\) to \(42\,mL\), the object's volume is:

13. Common mistakes to avoid

• Do not confuse mass and volume. Mass is how much matter is present. Volume is how much space it takes up.
• Do not forget units. A density answer should include units such as \(g/mL\) or \(g/cm^3\).
• Do not assume heavier objects always sink. A heavy object can float if its overall density is less than the liquid.
• Compare densities carefully. An object floats only if its density is less than the density of the liquid.

14. Quick review

• Density measures how much mass is in a certain volume.
• The density formula is $$d = \frac{m}{V}$$
• An object floats if it is less dense than the liquid.
• An object sinks if it is more dense than the liquid.
• Buoyancy is the upward force from a liquid.
• Archimedes' principle says buoyant force equals the weight of the displaced liquid.

Summary

Density and buoyancy work together to explain floating and sinking. Density compares mass to volume, while buoyancy is the upward push from a liquid. By calculating density and comparing it to the density of a liquid, you can predict whether an object will float, sink, or stay suspended. Archimedes' principle helps explain that the amount of liquid displaced determines the buoyant force on an object.

Separation Techniques are ways we can take apart a mixture. A mixture is made when two or more materials are together but are not changed into a new substance. This means we can often separate them again.

Scientists separate mixtures by looking at the physical properties of each part. Physical properties are things we can observe, like size, shape, weight, and whether something mixes with water. Different separation techniques work because the parts of a mixture are not exactly the same.

In this lesson, we will learn about four important separation techniques:

• Filtration
• Distillation
• Chromatography
• Centrifugation

Each method helps separate a mixture in a different way.

Why do we separate mixtures?

• To get something clean, like water
• To study what is inside a mixture
• To remove unwanted materials
• To collect useful parts

For example, if you have sand mixed with water, you may want the clean water, the dry sand, or both. A separation technique can help.

1. Filtration

Filtration is used to separate a solid from a liquid when the solid does not dissolve. In filtration, the mixture is poured through a filter. The filter has tiny holes.

The liquid can pass through the holes, but the larger solid pieces cannot. The solid stays behind, and the liquid moves through.

Filtration works because of particle size. If the solid pieces are bigger than the holes in the filter, they get trapped.

Examples of filtration:

• Separating sand from water
• Using a coffee filter to separate coffee grounds from liquid coffee
• Straining pasta from water with a colander

Important idea: Filtration does not work well if the solid is dissolved in the liquid. For example, you cannot use a filter to remove salt that has dissolved in water.

2. Distillation

Distillation is used to separate liquids or to separate a dissolved solid from a liquid. This method uses heating and cooling.

When a liquid is heated, it can change into a gas. This is called evaporation. If the gas is cooled, it turns back into a liquid. This is called condensation.

In distillation, one part of the mixture heats up and evaporates first. Then the gas is cooled and collected as a liquid in a different container.

Distillation works because different substances can change state at different temperatures.

Examples of distillation:

• Getting clean water from salty water
• Separating two liquids that heat up and evaporate at different temperatures

If salty water is heated, the water turns into vapor, but the salt stays behind. Then the vapor is cooled into liquid water again.

3. Chromatography

Chromatography is used to separate colors or other tiny parts that seem mixed together. It is often used with ink.

In a simple paper chromatography test, a spot of ink is placed on paper. Then the bottom of the paper is placed in water. The water moves up the paper and carries the colors with it.

Some colors move faster or farther than others. This happens because some colors mix with the water better, and some stick to the paper more.

Chromatography works because different parts of the mixture move at different speeds.

Examples of chromatography:

• Seeing the different colors inside a black marker
• Testing food coloring
• Comparing inks from different pens

4. Centrifugation

Centrifugation is a way to separate parts of a mixture by spinning it very fast. A machine called a centrifuge spins the mixture in a circle.

When the mixture spins, the heavier parts move outward and settle apart from the lighter parts. This helps separate things that are mixed together.

Centrifugation works because of difference in mass, or how heavy something is compared with something else.

Examples of centrifugation:

• Separating cream from milk
• Separating tiny solid bits from a liquid in a lab
• A washing machine spin cycle pushing water out of clothes

Even though a washing machine is not a lab centrifuge, it uses a similar spinning idea to help separate water from clothes.

How to choose the right separation technique

To choose the best method, ask questions about the mixture:

• Is there a solid that does not dissolve in a liquid? Use filtration.
• Is one part able to evaporate and then be collected again? Use distillation.
• Are you trying to separate colors or very tiny dissolved parts? Use chromatography.
• Do the parts have different masses and can be separated by spinning? Use centrifugation.

Worked Example 1: Sand and Water

Problem: Mia has a cup of sand mixed with water. She wants to separate the sand from the water.

Think: Sand does not dissolve in water. The sand pieces are bigger than the tiny holes in a filter.

Best method: Filtration

How it works:

1. Pour the sand-water mixture through filter paper or a strainer.
2. The water passes through.
3. The sand stays on the filter.

Answer: Filtration is the best way to separate sand and water.

Worked Example 2: Salty Water

Problem: Jay has water with salt dissolved in it. He wants to get the water back.

Think: The salt is dissolved, so filtration will not work. Water can evaporate and then condense back into liquid.

Best method: Distillation

How it works:

1. Heat the salty water.
2. The water changes into vapor.
3. The salt stays behind.
4. Cool the vapor so it turns back into liquid water.

Answer: Distillation separates the water from the salt.

Worked Example 3: Black Marker Colors

Problem: Elena wants to know if a black marker is really made of more than one color.

Think: She needs a method that can separate tiny mixed colors.

Best method: Chromatography

How it works:

1. Put a small dot of black marker on paper.
2. Place the bottom of the paper in water.
3. Watch the water move up the paper.
4. Different colors spread out to different places.

Answer: Chromatography can show the hidden colors in the marker.

Worked Example 4: Spinning to Separate

Problem: A scientist has a liquid with tiny heavy bits mixed in. The bits are so small that they do not come out easily with a filter.

Think: The scientist needs a way to separate heavier parts from lighter liquid by spinning.

Best method: Centrifugation

How it works:

1. Put the mixture into a centrifuge.
2. Spin it very fast.
3. The heavier bits move outward and settle apart.
4. The lighter liquid stays separated from them.

Answer: Centrifugation is the best choice.

Comparing the four methods

• Filtration: separates solids from liquids using a filter
• Distillation: separates using heating and cooling
• Chromatography: separates tiny dissolved parts, like colors, by how fast they move
• Centrifugation: separates by spinning mixtures very fast

Quick check questions

1. Which method would you use to separate mud from water?
2. Which method would help collect clean water from salty water?
3. Which method can show the different colors in ink?
4. Which method uses spinning to separate heavier and lighter parts?

Answers:

1. Filtration
2. Distillation
3. Chromatography
4. Centrifugation

Things to remember

• A mixture can often be separated because its parts keep their own physical properties.
• Different techniques work for different kinds of mixtures.
• You must look at the mixture carefully before choosing a method.

Lesson Summary

Separation techniques help us take apart mixtures using physical properties. Filtration separates solids from liquids, distillation uses heating and cooling, chromatography separates tiny mixed colors or dissolved parts, and centrifugation uses spinning. When you know the kind of mixture you have, you can choose the best way to separate it.

Pure Substances vs. Mixtures

Everything around us is made of matter. Matter is anything that has mass and takes up space. In science, we often group matter by asking an important question: Is it made of one kind of substance, or is it made of more than one substance mixed together?

This helps us sort matter into two big categories: pure substances and mixtures. Then we can break those groups into smaller categories: elements, compounds, homogeneous mixtures, and heterogeneous mixtures.

Learning these categories helps you describe materials correctly and understand how they behave.

1. What is a pure substance?

A pure substance is a kind of matter made of only one type of particle. It has a fixed composition, which means it is always made the same way.

For example, pure water is always water. Table sugar is always sugar. A piece of pure gold is only gold.

Pure substances are divided into two groups:

• Elements
• Compounds

2. Elements

An element is a pure substance made of only one kind of atom. It cannot be broken down into a simpler substance by ordinary chemical means.

Examples of elements include:

• Gold
• Oxygen
• Iron
• Helium

If you had a sample of pure oxygen, every particle in that sample would be oxygen atoms joined in the same way. If you had pure iron, it would contain only iron atoms.

3. Compounds

A compound is a pure substance made when two or more elements are chemically joined together in a fixed way.

This is important: in a compound, the elements are not just placed together. They are chemically bonded, which means they form a new substance with its own properties.

Examples of compounds include:

• Water, made from hydrogen and oxygen
• Carbon dioxide, made from carbon and oxygen
• Table salt, made from sodium and chlorine

Water is a compound because each water particle has the same ratio of atoms: $$H_2O$$. That means each particle has 2 hydrogen atoms and 1 oxygen atom.

Table salt is also a compound. It is not the same as just mixing sodium and chlorine together. Once they chemically combine, they make a new substance with different properties.

4. What is a mixture?

A mixture is matter made of two or more substances physically combined. The substances are not chemically bonded, so each substance keeps its own properties.

Mixtures do not have a fixed composition. This means the amount of each substance can change. For example, one glass of lemonade may have more sugar than another glass.

Examples of mixtures include:

• Salad
• Air
• Trail mix
• Salt water

Mixtures are divided into two groups:

• Homogeneous mixtures
• Heterogeneous mixtures

5. Homogeneous mixtures

A homogeneous mixture is a mixture that is evenly mixed throughout. The different parts are spread out so well that the mixture looks like it is all one substance.

You usually cannot easily see the different substances in a homogeneous mixture.

Examples include:

• Salt dissolved in water
• Air
• Lemonade with the sugar fully dissolved

If you taste salt water from different parts of the glass, it should taste about the same everywhere. That is because the salt is evenly spread throughout the water.

6. Heterogeneous mixtures

A heterogeneous mixture is a mixture that is not evenly mixed. You can often see the different parts or substances.

Examples include:

• Salad
• Trail mix
• Sand and water
• Cereal in milk

If you look at trail mix, you can see the raisins, nuts, and chocolate pieces. They are mixed together, but each part still looks different.

7. How to tell the categories apart

When you are trying to classify matter, ask these questions in order:

1. Is it made of one substance or more than one substance?
2. If it is one substance, is it an element or a compound?
3. If it is more than one substance, is it mixed evenly or unevenly?

Here is a simple guide:

• One kind of atom only  element
• Two or more elements chemically joined  compound
• Two or more substances evenly mixed  homogeneous mixture
• Two or more substances unevenly mixed  heterogeneous mixture

8. Pure substances and mixtures compared

These differences are very important:

• Pure substances have a fixed composition.
• Mixtures can have different amounts of each substance.
• Compounds are chemically joined.
• Mixtures are physically combined.
• Homogeneous mixtures look uniform.
• Heterogeneous mixtures do not look uniform.

9. Worked Example 1: Classifying oxygen

Question: Is oxygen an element, compound, homogeneous mixture, or heterogeneous mixture?

Step 1: Ask if it is one substance or more than one substance. Pure oxygen is only oxygen, so it is one substance.

Step 2: Ask if it is made of one kind of atom or different kinds of atoms chemically joined. Oxygen is made of only oxygen atoms.

Answer: Oxygen is an element.

10. Worked Example 2: Classifying water

Question: Is pure water an element, compound, homogeneous mixture, or heterogeneous mixture?

Step 1: Pure water is one substance, so it is a pure substance.

Step 2: Water is made from hydrogen and oxygen chemically joined in the ratio $$2:1$$.

Answer: Pure water is a compound.

11. Worked Example 3: Classifying salt water

Question: Is salt water an element, compound, homogeneous mixture, or heterogeneous mixture?

Step 1: Salt water has more than one substance: salt and water.

Step 2: The salt is physically mixed with the water, not turned into a new substance.

Step 3: If the salt is fully dissolved, the mixture looks the same throughout.

Answer: Salt water is a homogeneous mixture.

12. Worked Example 4: Classifying trail mix

Question: Is trail mix an element, compound, homogeneous mixture, or heterogeneous mixture?

Step 1: Trail mix contains more than one substance, such as nuts, raisins, and chocolate pieces.

Step 2: These parts are physically combined, not chemically bonded.

Step 3: You can clearly see the different parts, so it is not evenly mixed.

Answer: Trail mix is a heterogeneous mixture.

13. Common mistakes to avoid

• Mistake 1: Thinking every substance with more than one atom is a compound. A substance can still be an element if it has only one kind of atom.
• Mistake 2: Thinking dissolved substances disappear. In salt water, the salt is still there; it is just spread out evenly.
• Mistake 3: Confusing compounds with mixtures. In compounds, substances are chemically joined. In mixtures, they are only physically combined.
• Mistake 4: Thinking all mixtures look different. Some mixtures, like air and salt water, look uniform and are homogeneous.

14. Quick practice ideas

Try classifying these on your own:

• Helium
• Carbon dioxide
• Air
• Sand in water

Answers:

• Helium  element
• Carbon dioxide  compound
• Air  homogeneous mixture
• Sand in water  heterogeneous mixture

15. Summary

Matter can be grouped as pure substances or mixtures. Pure substances have a fixed composition and include elements and compounds. Mixtures are made of substances physically combined and include homogeneous mixtures, which are evenly mixed, and heterogeneous mixtures, which are not evenly mixed.

If you remember to ask whether a material is one substance or more than one, and then whether it is chemically joined or physically combined, you will be able to classify it correctly.

Solutions and Solubility

Have you ever stirred sugar into tea or mixed powdered drink mix into water? When one substance spreads out evenly in another, it forms a solution. Solutions are an important part of science because many materials around us are mixtures that look like a single substance.

In this lesson, you will learn what solutions are, what solute and solvent mean, what solubility means, how to describe concentration, and how temperature can affect how much of a substance dissolves.

1. What is a solution?

A solution is a mixture in which one substance is evenly dissolved in another. Because the particles are spread out evenly, you usually cannot see the different parts.

Every solution has two main parts:

• Solute – the substance that gets dissolved
• Solvent – the substance that does the dissolving

For example, if you mix salt into water:

• Salt is the solute
• Water is the solvent

The result is a saltwater solution.

2. How do particles behave in a solution?

All matter is made of tiny particles. When a solute dissolves, its particles separate and spread out between the particles of the solvent.

For example, when sugar dissolves in water, the sugar particles mix evenly with the water particles. The sugar seems to “disappear,” but it is still there. You can prove this because the water tastes sweet.

This is different from a mixture like sand in water. Sand does not dissolve. Its particles stay large enough to see, and they may sink to the bottom.

3. Soluble and insoluble

A substance is soluble if it can dissolve in a certain solvent. A substance is insoluble if it does not dissolve well in that solvent.

Examples:

• Sugar is soluble in water.
• Salt is soluble in water.
• Sand is insoluble in water.
• Oil does not mix well with water, so it is not soluble in water.

A substance may be soluble in one solvent but not in another. So when we talk about solubility, we must ask: soluble in what?

4. What is solubility?

Solubility is the amount of solute that can dissolve in a certain amount of solvent at a certain temperature.

This means solubility depends on:

• What the solute is
• What the solvent is
• The temperature

For example, a certain amount of water can dissolve only a certain maximum amount of sugar at room temperature. If you keep adding sugar, eventually no more will dissolve.

5. Unsaturated, saturated, and supersaturated

Scientists use special words to describe how much solute is dissolved in a solution.

• Unsaturated solution – more solute can still dissolve
• Saturated solution – the maximum amount of solute has dissolved at that temperature
• Supersaturated solution – contains more dissolved solute than is normally possible at that temperature

For 7th grade, the most important ideas are unsaturated and saturated.

If you stir one spoonful of salt into water and it all disappears, the solution may still be unsaturated. If you add more and more salt until some stays at the bottom, the solution is likely saturated.

6. Concentration

Concentration tells how much solute is dissolved in a given amount of solution or solvent.

A solution can be described as:

• Dilute – a small amount of solute is dissolved
• Concentrated – a larger amount of solute is dissolved

Imagine two cups of water:

• Cup A has 1 spoonful of drink mix.
• Cup B has 4 spoonfuls of drink mix.

Cup B is more concentrated because it has more solute in about the same amount of solvent.

Sometimes concentration can be compared using simple ratios. For example, if 10 grams of salt are dissolved in 100 mL of water, the concentration is greater than if 5 grams are dissolved in 100 mL of water.

7. Temperature and solubility

For many solid solutes, solubility increases as temperature increases. This means hot water can often dissolve more solid material than cold water.

For example, hot tea usually dissolves sugar faster and can hold more sugar than iced tea.

This happens because the particles in warmer liquids move faster. Faster-moving particles can help break apart the solute and spread it through the solvent.

However, not every substance behaves exactly the same way. Still, for many solids in water, a higher temperature means more solute can dissolve.

8. Solubility curves

A solubility curve is a graph that shows how much solute can dissolve in a certain amount of solvent at different temperatures.

Usually:

• The x-axis shows temperature.
• The y-axis shows the amount of solute that dissolves.

If a point is on the curve, the solution is saturated at that temperature.

If a point is below the curve, the solution is unsaturated.

If a point is above the curve, more solute is present than should normally dissolve at that temperature.

Here is a simple example of reading a solubility curve:

• At \(20^\circ C\), a solute might have a solubility of 30 g in 100 g of water.
• At \(40^\circ C\), the same solute might have a solubility of 50 g in 100 g of water.

This tells us that warmer water can dissolve more of that solute.

9. Worked Example 1: Identify solute and solvent

Problem: A student mixes 2 spoonfuls of sugar into a cup of hot water. What are the solute and the solvent?

Step 1: Find the substance being dissolved. That is the sugar.

Step 2: Find the substance doing the dissolving. That is the water.

Answer:

• Solute: sugar
• Solvent: water

10. Worked Example 2: Compare concentration

Problem: Beaker A has 5 g of salt dissolved in 100 mL of water. Beaker B has 15 g of salt dissolved in 100 mL of water. Which solution is more concentrated?

Step 1: Compare the amount of solute. Beaker A has 5 g, and Beaker B has 15 g.

Step 2: The amount of water is the same in both beakers, so the beaker with more solute is more concentrated.

Answer: Beaker B is more concentrated.

11. Worked Example 3: Decide if a solution is saturated

Problem: At a certain temperature, 20 g of sugar is the maximum amount that can dissolve in 100 g of water. A student adds 18 g of sugar to 100 g of water, and it all dissolves. Is the solution unsaturated or saturated?

Step 1: Compare the amount added to the maximum solubility.

The maximum is 20 g, but only 18 g was added.

Step 2: Since the solution has less than the maximum amount, more sugar could still dissolve.

Answer: The solution is unsaturated.

12. Worked Example 4: Read a simple solubility curve idea

Problem: A solubility graph shows that a substance dissolves up to 40 g in 100 g of water at \(30^\circ C\). If a student puts 50 g of the substance into 100 g of water at \(30^\circ C\), what happens?

Step 1: Find the maximum amount that can dissolve at that temperature. The graph says 40 g.

Step 2: Compare the amount added. The student added 50 g, which is 10 g too much.

We can show the extra amount with subtraction:

$$50 - 40 = 10$$

Step 3: Only 40 g dissolves. The extra 10 g does not dissolve and may remain at the bottom.

Answer: The solution becomes saturated, and 10 g stays undissolved.

13. Factors that affect dissolving

Besides temperature, there are other things that can affect how fast a solute dissolves:

• Stirring – helps spread particles through the solvent
• Crushing – smaller pieces dissolve faster because more surface is exposed
• Heating – often helps solids dissolve faster and sometimes allows more to dissolve

These factors often affect the speed of dissolving. Temperature can also affect the amount that dissolves.

14. Everyday examples of solutions

• Saltwater
• Sugar water
• Lemonade mix in water
• Tea with dissolved sugar
• Sports drinks

Many liquids we drink are solutions because different substances are dissolved evenly in water.

15. Common mistakes to avoid

• Mistake: Thinking the solute disappears forever.
  Correct idea: The solute is still present; its particles are just spread out.
• Mistake: Confusing dissolving with melting.
  Correct idea: Dissolving means mixing into a solvent. Melting means changing from solid to liquid because of heat.
• Mistake: Thinking all substances dissolve in water.
  Correct idea: Some are soluble in water, and some are insoluble.
• Mistake: Thinking a concentrated solution must be saturated.
  Correct idea: A solution can be concentrated but still able to dissolve more solute.

16. Quick review

• A solution is a mixture where one substance dissolves evenly in another.
• The solute is dissolved.
• The solvent does the dissolving.
• Solubility is the amount of solute that can dissolve in a certain amount of solvent at a certain temperature.
• Concentration tells how much solute is in a solution.
• Unsaturated means more can dissolve.
• Saturated means the maximum amount has dissolved at that temperature.
• For many solids, increasing temperature increases solubility.
• A solubility curve shows how solubility changes with temperature.

17. Summary

Solutions form when a solute dissolves evenly in a solvent. Solubility tells how much of a solute can dissolve at a certain temperature, and concentration tells how much solute is present in the solution.

As temperature rises, many solid substances dissolve more easily and in greater amounts. By understanding solute, solvent, concentration, and solubility curves, you can explain how and why different solutions behave the way they do.

Separation Techniques are ways to split a mixture into its parts. A mixture is made of two or more substances that are together but not chemically joined. Because the substances keep their own physical properties, we can often separate them.

Scientists use separation techniques to clean water, make medicines, test food, and study unknown materials. In everyday life, people also separate mixtures when they strain pasta, filter coffee, or spin wet clothes in a washing machine.

To choose the best separation method, we look at how the substances are different. These differences can include:

• Particle size
• Boiling point
• Density
• How well a substance dissolves
• How strongly a substance moves with a liquid

In this lesson, you will learn four important separation techniques:

• Filtration
• Distillation
• Centrifugation
• Chromatography

1. Filtration

Filtration is used to separate an insoluble solid from a liquid. Insoluble means the solid does not dissolve in the liquid.

In filtration, the mixture is poured through a filter. The filter has tiny holes. The liquid can pass through, but the larger solid particles get trapped.

The solid left behind on the filter is called the residue. The liquid that passes through is called the filtrate.

Filtration works because of a difference in particle size. The solid particles are too large to fit through the holes in the filter.

Examples of filtration:

• Separating sand from water
• Making coffee with a coffee filter
• Cleaning dirty water by trapping solid particles

Important note: Filtration does not separate substances that are dissolved. For example, salt dissolved in water will pass through the filter with the water.

2. Distillation

Distillation is used to separate substances in a liquid mixture by using differences in boiling point. The boiling point is the temperature at which a liquid changes to a gas.

When a mixture is heated, the substance with the lower boiling point boils first. Its vapor can be cooled and turned back into a liquid. This liquid is then collected separately.

This process has two main steps:

1. Evaporation — the liquid changes into gas when heated.
2. Condensation — the gas cools and changes back into liquid.

Distillation is useful when liquids have different boiling points, or when you want to separate a dissolved solid from a liquid and collect the liquid.

Examples of distillation:

• Getting pure water from salty water
• Separating two liquids with different boiling points
• Making some perfumes and fuels

If salt water is heated, the water evaporates first because its boiling point is lower than the salt's. The salt stays behind, and the water vapor can be condensed into pure liquid water.

3. Centrifugation

Centrifugation is a method that separates parts of a mixture by spinning it very fast. This works because substances with different densities move to different places when spun.

Density tells how much mass is packed into a certain volume. A simple way to describe density is:

$$\text{density} = \frac{\text{mass}}{\text{volume}}$$

When a mixture spins in a centrifuge, the denser particles move outward and settle at the bottom of the tube. The less dense part stays above.

This method is especially useful when particles are too small to separate easily by just waiting, or when filtration would be slow or difficult.

Examples of centrifugation:

• Separating blood into parts
• Separating cream from milk
• Removing water from wet clothes during the spin cycle

After centrifugation, the heavy material collected at the bottom is often called the pellet, and the liquid above it is called the supernatant.

4. Chromatography

Chromatography is used to separate substances that are dissolved in a liquid. It is often used to separate colored substances, like inks, dyes, or plant pigments.

In simple paper chromatography, a small spot of the mixture is placed near the bottom of a strip of paper. The bottom of the paper is placed in a solvent, such as water or alcohol. The solvent moves upward through the paper.

As the solvent travels, it carries the substances with it. Some substances move farther than others. This happens because each substance has a different attraction to the paper and to the solvent.

If one color of ink is really a mixture of several dyes, chromatography can separate it into different colored spots.

Examples of chromatography:

• Finding out which dyes are in black marker ink
• Testing food coloring
• Separating pigments in leaves

How to choose the right separation technique

To decide which method to use, ask these questions:

• Is the solid dissolved or not dissolved?
• Do the substances have different boiling points?
• Do the substances have different densities?
• Are the substances colored or able to travel differently with a solvent?

Use this guide:

• Filtration — use for an insoluble solid and a liquid
• Distillation — use for liquids with different boiling points, or to get a liquid back from a solution
• Centrifugation — use for substances with different densities, especially very small particles in a liquid
• Chromatography — use for dissolved substances that move differently through a material

Comparing the four methods

• Filtration depends on particle size.
• Distillation depends on boiling point.
• Centrifugation depends on density.
• Chromatography depends on how substances move differently with a solvent and a surface like paper.

Worked Example 1: Sand and water

Problem: A student has a mixture of sand and water. What is the best way to separate them?

Step 1: Decide what kind of mixture this is. Sand does not dissolve in water, so it is an insoluble solid mixed with a liquid.

Step 2: Choose the method. Filtration is best for separating an insoluble solid from a liquid.

Answer: Use filtration. The sand stays on the filter as the residue, and the water passes through as the filtrate.

Worked Example 2: Salt water

Problem: A student wants to get pure water from salt water. Which separation method should be used?

Step 1: Notice that the salt is dissolved in the water. Filtration will not work because the salt particles are too small and move through with the water.

Step 2: Think about boiling points. Water can boil and turn into vapor, while salt stays behind.

Step 3: Collect the water vapor and cool it back into liquid.

Answer: Use distillation to collect pure water.

Worked Example 3: Blood sample

Problem: A lab needs to separate the heavier parts of blood from the liquid part. What method should be used?

Step 1: Think about the property that is different. The parts of blood have different densities.

Step 2: Choose the method that separates by density.

Answer: Use centrifugation. Spinning causes the denser parts to settle lower in the tube.

Worked Example 4: Black ink

Problem: A teacher wants to know whether black ink is made from only one dye or from several dyes. What method should be used?

Step 1: The dyes are dissolved substances.

Step 2: The goal is to see whether the dissolved substances travel different distances in a solvent.

Answer: Use chromatography. If the ink separates into different colored spots, it contains more than one dye.

Common mistakes to avoid

• Do not choose filtration for a dissolved substance like salt in water.
• Do not forget that distillation needs heating and cooling.
• Do not confuse centrifugation with filtration. Centrifugation separates by density, not by a paper filter.
• Do not think chromatography is only for colors. It is often used for colors, but the main idea is that substances move differently with a solvent.

Why separation techniques matter

Separation techniques are important in science, medicine, and daily life. They help us purify materials, test samples, recycle resources, and understand what a mixture contains.

Learning these methods also helps you see that the properties of matter are useful. By observing physical properties, scientists can choose the best way to separate and study substances.

Summary

Mixtures can be separated because their parts have different physical properties. Filtration separates insoluble solids from liquids by particle size. Distillation separates substances by boiling point. Centrifugation separates by density, and chromatography separates dissolved substances based on how they move with a solvent.

Acids, Bases, and pH

Everything around us is made of matter. Some kinds of matter are liquids we drink, foods we eat, or cleaners we use. These things can be sorted into groups called acids and bases.

Acids and bases are special kinds of substances. They can act in different ways. Scientists use a tool called the pH scale to help sort them.

In this lesson, you will learn what acids and bases are, what the pH scale looks like, and how to tell if something is an acid, a base, or in the middle.

What Is an Acid?

An acid is a substance that is often sour. Some acids are safe to eat, like the acid in lemons and oranges. Other acids are not for eating and should only be handled by adults.

Here are some things acids may be like:

• They can taste sour.
• Some are found in fruits.
• They are on one side of the pH scale.

Examples of acids:

• Lemon juice
• Orange juice
• Vinegar

What Is a Base?

A base is a substance that is often slippery or soapy. Many cleaners are bases. We do not taste bases because many are not safe to eat.

Here are some things bases may be like:

• They can feel slippery.
• Many soaps and cleaners are bases.
• They are on the other side of the pH scale.

Examples of bases:

• Soapy water
• Baking soda mixed with water
• Some cleaning liquids

What Is the pH Scale?

The pH scale is a number line that helps us sort substances. It goes from 0 to 14.

We can write it like this:

$$0 \quad 1 \quad 2 \quad 3 \quad 4 \quad 5 \quad 6 \quad 7 \quad 8 \quad 9 \quad 10 \quad 11 \quad 12 \quad 13 \quad 14$$

On the pH scale:

• Numbers below 7 are acids.
• The number 7 is neutral.
• Numbers above 7 are bases.

Neutral means not an acid and not a base. Plain water is close to neutral.

We can think about it like this:

• Acid: pH less than 7, or \(pH < 7\)
• Neutral: pH equals 7, or \(pH = 7\)
• Base: pH more than 7, or \(pH > 7\)

How Do We Use the pH Scale?

Scientists can use special paper or tools to test pH. The test can show a number or a color. Then they match the result to the pH scale.

If the number is small, like 2 or 3, the substance is an acid. If the number is large, like 10 or 11, the substance is a base. If the number is 7, it is neutral.

Acids and Bases in Everyday Life

You can find acids and bases in many places.

• Lemon juice is an acid.
• Vinegar is an acid.
• Water is neutral.
• Soapy water is a base.
• Baking soda and water can be a base.

It is important to remember that we should not taste or touch unknown things. Adults and scientists use safe tools and follow safety rules.

Worked Example 1: Find the Acid

A cup of lemon juice has a pH of 2. Is it an acid, a base, or neutral?

Step 1: Look at the number 2.

Step 2: Ask, is 2 below 7?

Yes, \(2 < 7\).

Answer: Lemon juice is an acid.

Worked Example 2: Find the Neutral Substance

A glass of plain water has a pH of 7. Is it an acid, a base, or neutral?

Step 1: Look at the number 7.

Step 2: Ask, is it exactly 7?

Yes, \(7 = 7\).

Answer: Plain water is neutral.

Worked Example 3: Find the Base

Soapy water has a pH of 10. Is it an acid, a base, or neutral?

Step 1: Look at the number 10.

Step 2: Ask, is 10 above 7?

Yes, \(10 > 7\).

Answer: Soapy water is a base.

Worked Example 4: Sort Three Substances

Here are three pH numbers:

• Vinegar: 3
• Water: 7
• Baking soda water: 9

Let us sort them.

Step 1: Numbers below 7 are acids.

Vinegar has pH 3, so vinegar is an acid.

Step 2: Number 7 is neutral.

Water has pH 7, so water is neutral.

Step 3: Numbers above 7 are bases.

Baking soda water has pH 9, so it is a base.

Easy Way to Remember

• Acids are below 7.
• Neutral is 7.
• Bases are above 7.

Safety Reminder

Do not taste, touch, or smell unknown liquids. Some acids and bases can hurt your skin or eyes. Always ask an adult before doing a science test.

Summary

Acids and bases are two groups of substances. The pH scale helps us sort them using numbers from 0 to 14.

If the pH is below 7, the substance is an acid. If the pH is 7, it is neutral. If the pH is above 7, it is a base.

Examples of acids are lemon juice and vinegar. Examples of bases are soapy water and baking soda water. Plain water is neutral.

Evolution of Atomic Theory is the story of how scientists slowly changed their ideas about what atoms are like. Today, we know that all matter is made of tiny atoms, but people did not always understand atoms the way we do now.

As scientists did experiments and collected evidence, they improved their atomic models. A model is an idea or drawing that helps explain how something works. Each new model of the atom was better than the one before it because it explained more evidence.

In this lesson, you will learn how atomic theory changed from simple solid spheres to a more complete model with a nucleus, energy levels, and an electron cloud.

Why atomic theory changed over time

Science is not just a list of facts. Science changes when new evidence is discovered. When scientists found results that older atomic models could not explain, they created new models.

This means atomic theory evolved. It did not change because the older scientists were careless. It changed because tools, experiments, and understanding improved.

1. Early idea: Democritus

Long ago, a Greek thinker named Democritus suggested that matter could be broken into smaller and smaller pieces until you reached a tiny piece that could not be divided anymore. He called these tiny pieces atomos, which means "cannot be cut."

Democritus had an important idea, but he did not have experiments to prove it. Because there was no evidence, his idea was not accepted as science at the time.

2. Dalton's atomic theory

In the early 1800s, John Dalton used experiments to support the idea that matter is made of atoms. This was the first modern atomic theory.

Dalton's model described atoms as tiny, solid spheres, like very small balls. This is often called the solid sphere model.

Dalton's main ideas were:

• All matter is made of atoms.
• Atoms of the same element are alike.
• Atoms of different elements are different.
• Atoms combine in simple whole-number ratios to form compounds.
• In chemical reactions, atoms are rearranged, not created or destroyed.

Dalton's model was a big step forward because it used evidence from experiments. However, later scientists discovered that atoms are not solid, indivisible balls after all.

3. Thomson and the electron

In the late 1800s, J. J. Thomson performed experiments with cathode rays. From these experiments, he discovered the electron, a tiny particle with a negative charge.

This discovery was very important. If atoms contain electrons, then atoms are not indivisible solid spheres. They must have smaller parts inside them.

Thomson proposed the plum pudding model. In this model, the atom was a positively charged sphere with negatively charged electrons scattered throughout it.

You can imagine it like a ball of dough with small bits mixed in. Even though this model was later replaced, it was important because it showed that atoms have internal structure.

4. Rutherford and the nucleus

In 1911, Ernest Rutherford tested Thomson's model using the famous gold foil experiment.

He shot tiny positively charged particles at a thin sheet of gold foil. According to Thomson's model, the particles should have mostly passed straight through with only small changes in direction.

Most particles did pass through. But a few bounced back or were strongly deflected. This surprised scientists.

Rutherford concluded that:

• Most of the atom is empty space.
• Almost all of the atom's mass is packed into a tiny, dense center.
• This center is called the nucleus.

Rutherford's model replaced Thomson's model. In Rutherford's model, electrons moved around a small, positive nucleus.

This was a huge improvement because it explained the gold foil experiment. Still, scientists needed a better explanation for how electrons moved around the nucleus.

5. Bohr and energy levels

In 1913, Niels Bohr improved Rutherford's model. Bohr said that electrons move around the nucleus in specific paths or energy levels.

According to the Bohr model:

• Electrons are found in fixed energy levels around the nucleus.
• Electrons can move from one energy level to another by gaining or losing energy.
• Electrons closer to the nucleus have less energy than electrons farther away.

This model helped explain why atoms give off light in certain colors. When an electron moves down to a lower energy level, it releases energy.

For 7th grade, it is enough to understand that Bohr's model organized electrons into levels around the nucleus. This model is still often used in simple drawings of atoms because it is easy to understand.

6. The quantum mechanical model

Later, scientists learned that electrons do not move in neat circular paths exactly like planets orbiting the Sun. Instead, the modern model describes electrons as moving in regions around the nucleus called an electron cloud.

This modern idea is called the quantum mechanical model. In this model, scientists cannot always say the exact path of an electron. Instead, they can describe the area where an electron is most likely to be found.

For 7th grade, the most important idea is this: electrons move around the nucleus in a cloud-like region, not in perfect circles.

7. Chadwick and the neutron

Another important discovery was made by James Chadwick in 1932. He discovered the neutron, a particle in the nucleus with no charge.

Now scientists understood that the nucleus contains:

• Protons, which have a positive charge
• Neutrons, which have no charge

And outside the nucleus are:

• Electrons, which have a negative charge

This helped complete the modern picture of the atom.

How the models changed

Each scientist built on earlier ideas. The atomic model did not change all at once. It changed step by step as new evidence was found.

1. Democritus: Matter is made of tiny particles called atomos.
2. Dalton: Atoms are solid spheres; different elements have different atoms.
3. Thomson: Atoms contain electrons; plum pudding model.
4. Rutherford: Atoms have a tiny dense nucleus and are mostly empty space.
5. Bohr: Electrons occupy specific energy levels around the nucleus.
6. Quantum mechanical model: Electrons move in an electron cloud.
7. Chadwick: The nucleus also contains neutrons.

Why each model mattered

• Dalton made atomic theory scientific by using experiments.
• Thomson proved atoms have smaller parts.
• Rutherford discovered the nucleus.
• Bohr explained energy levels of electrons.
• The modern model gave a more accurate picture of electron movement.
• Chadwick explained the missing particle in the nucleus.

Important particles in the atom

• Proton: positive charge, found in the nucleus
• Neutron: no charge, found in the nucleus
• Electron: negative charge, found in the electron cloud

An atom is usually neutral, which means it has equal numbers of protons and electrons.

For example, if an atom has 6 protons, it usually has 6 electrons. The positive and negative charges balance.

Worked Example 1: Putting the models in order

Question: Put these atomic models in order from oldest to newest: Bohr, Dalton, Rutherford, Thomson, quantum mechanical model.

Step 1: Remember the timeline of discoveries.

Dalton came first with the solid sphere model. Thomson came next after discovering electrons. Rutherford followed with the nucleus. Bohr then added energy levels. Finally, the quantum mechanical model described the electron cloud.

Answer: Dalton  Thomson  Rutherford  Bohr  quantum mechanical model.

Worked Example 2: Matching evidence to a scientist

Question: A scientist found that most of the atom is empty space and that it has a small, dense center. Which scientist does this describe?

Step 1: Think about which experiment showed particles passing through foil but a few bouncing back.

Step 2: That was the gold foil experiment.

Step 3: The scientist who used that experiment was Ernest Rutherford.

Answer: Rutherford.

Worked Example 3: Comparing models

Question: How is Bohr's model different from Rutherford's model?

Step 1: Rutherford said electrons move around the nucleus, but his model did not clearly explain how they were arranged.

Step 2: Bohr added the idea that electrons are in specific energy levels.

Answer: Bohr's model improved Rutherford's by showing that electrons occupy specific energy levels around the nucleus.

Worked Example 4: Identifying the modern model

Question: A diagram shows a nucleus with a fuzzy cloud around it instead of rings. Which atomic model is this?

Step 1: Rings usually represent the Bohr model.

Step 2: A fuzzy cloud represents where electrons are likely to be found.

Answer: This is the quantum mechanical model, also called the electron cloud model.

Common mistakes to avoid

• Do not say Dalton discovered electrons. Thomson discovered electrons.
• Do not say Rutherford discovered energy levels. Bohr introduced energy levels.
• Do not think electrons are in the nucleus. Electrons are outside the nucleus.
• Do not think the atom is solid all the way through. Most of an atom is empty space.
• Do not confuse the Bohr model with the modern model. Bohr used fixed levels, while the modern model uses an electron cloud.

Big idea

The evolution of atomic theory shows how science grows. Scientists ask questions, test ideas, and improve models when new evidence is found.

Each atomic model was useful in its time, but newer models explained more. Because of this work, we now understand atoms as having a dense nucleus made of protons and neutrons, with electrons moving in an electron cloud around it.

Brief Summary

Atomic theory changed over time as scientists learned more from experiments. Dalton described atoms as solid spheres, Thomson discovered electrons, Rutherford discovered the nucleus, Bohr added energy levels, Chadwick discovered neutrons, and the modern quantum mechanical model describes electrons as moving in an electron cloud. This timeline shows how science improves when new evidence is found.

Subatomic Particles are the tiny particles that make up an atom. Everything around you is made of matter, and all matter is made of atoms. To understand atoms, we need to learn about the three main subatomic particles: protons, neutrons, and electrons.

These particles are important because they help explain an atom’s mass, charge, and structure. In this lesson, you will learn what each particle is, where it is found, and how it affects the atom.

First, let’s look at the inside of an atom. At the center of an atom is a small, dense region called the nucleus. The nucleus contains protons and neutrons. Moving around the outside of the nucleus are electrons.

Even though atoms are very tiny, most of the atom’s mass is packed into the nucleus. That is because protons and neutrons have much more mass than electrons.

The three main subatomic particles are:

• Proton: positive charge, found in the nucleus
• Neutron: no charge, found in the nucleus
• Electron: negative charge, found outside the nucleus

1. Protons

A proton is a subatomic particle with a positive charge. We show its charge as \,\(+1\). Protons are located in the nucleus of the atom.

The number of protons in an atom is very important. It tells us what element the atom is. For example, every hydrogen atom has 1 proton, and every carbon atom has 6 protons.

Protons also have mass. In 7th grade science, we usually say a proton has a relative mass of 1.

2. Neutrons

A neutron is a subatomic particle with no charge. We can say its charge is \,\(0\). Neutrons are also found in the nucleus.

Neutrons help add mass to the atom. Like protons, a neutron has a relative mass of about 1.

Neutrons are important because they help make the nucleus more stable. At this level, you can remember that neutrons add mass but do not change the atom’s charge.

3. Electrons

An electron is a subatomic particle with a negative charge. We show its charge as \,\(-1\). Electrons are found outside the nucleus in the space around it.

Electrons have very little mass compared to protons and neutrons. In middle school science, we often say their relative mass is about 0 because it is so small.

Even though electrons have very little mass, they are very important. Electrons affect how atoms interact with each other and how they form bonds.

Comparing the three particles

Here is a simple way to compare charge, mass, and location:

• Proton: charge \,\(+1\), mass \,\(1\), location: nucleus
• Neutron: charge \,\(0\), mass \,\(1\), location: nucleus
• Electron: charge \,\(-1\), mass \,\(0\) relative, location: outside nucleus

How charge works in an atom

Charges can balance each other. A positive proton and a negative electron have equal but opposite charges. In a neutral atom, the number of protons equals the number of electrons.

For example, if an atom has 6 protons, it must also have 6 electrons to have no overall charge. The positive and negative charges balance.

We can think of total charge like this:

$$\text{total charge} = (\text{number of protons}) - (\text{number of electrons})$$

Neutrons are not included in the charge calculation because they have no charge.

How mass works in an atom

Almost all of the atom’s mass comes from the protons and neutrons in the nucleus. Since each proton and neutron has a relative mass of about 1, we can estimate atomic mass by adding them together.

$$\text{mass number} = \text{protons} + \text{neutrons}$$

Electrons are not usually included in this calculation because their mass is so small.

Why location matters

The nucleus is at the center of the atom and contains protons and neutrons. It is very small but holds most of the atom’s mass.

The electrons move in the space around the nucleus. This means the atom has a dense center and a larger outer region where electrons are found.

A simple model of the atom is:

• Center: nucleus with protons and neutrons
• Outside region: electrons

Worked Example 1: Identifying each particle

Question: Which subatomic particle has a negative charge and is found outside the nucleus?

Step 1: Look for the particle with charge \,\(-1\).

Step 2: Check where it is located.

Answer: The electron has a negative charge and is found outside the nucleus.

Worked Example 2: Finding total charge of a neutral atom

Question: An atom has 8 protons. If it is neutral, how many electrons does it have?

Step 1: A neutral atom has equal numbers of protons and electrons.

Step 2: The atom has 8 protons, so it must have 8 electrons.

Answer: The atom has 8 electrons.

Worked Example 3: Finding mass number

Question: An atom has 11 protons and 12 neutrons. What is its mass number?

Step 1: Use the formula

$$\text{mass number} = \text{protons} + \text{neutrons}$$

Step 2: Substitute the numbers.

$$11 + 12 = 23$$

Answer: The mass number is 23.

Worked Example 4: Using charge and location together

Question: A particle is in the nucleus and has no charge. What particle is it, and what is its relative mass?

Step 1: A particle in the nucleus could be a proton or a neutron.

Step 2: The particle has no charge, so it must be a neutron.

Step 3: A neutron has a relative mass of about 1.

Answer: It is a neutron, and its relative mass is 1.

Common mistakes to avoid

• Do not confuse protons and electrons. Protons are positive and in the nucleus. Electrons are negative and outside the nucleus.
• Do not forget that neutrons have no charge.
• Do not include electrons when finding mass number in basic calculations.
• Do not say electrons are in the nucleus. They are found outside the nucleus.

Helpful memory clues

• Proton = Positive
• Neutron = Neutral
• Electron = outside the nucleus, with negative charge

Quick review

1. Atoms are made of protons, neutrons, and electrons.
2. Protons and neutrons are in the nucleus.
3. Electrons are outside the nucleus.
4. Protons are positive, neutrons are neutral, and electrons are negative.
5. Protons and neutrons each have a relative mass of about 1.
6. Electrons have very little mass compared to the other two particles.

Summary

Subatomic particles are the parts of an atom. Protons have a positive charge, neutrons have no charge, and electrons have a negative charge. Protons and neutrons are found in the nucleus, while electrons are found outside the nucleus. Protons and neutrons make up most of the atom’s mass, and the number of protons and electrons helps determine the atom’s overall charge.

Atomic Number and Mass Number

Everything around us is made of matter, and matter is made of tiny particles called atoms. Even though atoms are very small, scientists have learned a lot about what is inside them.

To understand atoms, it helps to know the three main particles inside them:

• Protons - found in the center of the atom, called the nucleus
• Neutrons - also found in the nucleus
• Electrons - found outside the nucleus

In this lesson, we will focus on two important ideas: atomic number and mass number. These numbers help us identify atoms and describe what they are made of.

1. What is the atomic number?

The atomic number tells how many protons are in the nucleus of an atom.

This is very important because the number of protons tells us which element the atom is. Every element has its own atomic number.

• If an atom has 1 proton, it is hydrogen.
• If an atom has 6 protons, it is carbon.
• If an atom has 8 protons, it is oxygen.

No two different elements have the same atomic number. That means the atomic number is like an element's ID number.

We can write this idea as:

$$\text{Atomic Number} = \text{Number of Protons}$$

2. What is the mass number?

The mass number tells the total number of particles in the nucleus.

Since the nucleus contains only protons and neutrons, the mass number is found by adding them together.

$$\text{Mass Number} = \text{Number of Protons} + \text{Number of Neutrons}$$

Electrons are not included in the mass number because they are outside the nucleus and have much less mass than protons and neutrons.

3. Why are these numbers different?

Students sometimes confuse atomic number and mass number because both use numbers about the atom. But they mean different things.

• Atomic number counts only protons.
• Mass number counts protons + neutrons.

So, the mass number is usually larger than the atomic number.

4. Finding neutrons

If you know the atomic number and mass number, you can figure out the number of neutrons.

Start with:

$$\text{Mass Number} = \text{Protons} + \text{Neutrons}$$

Then subtract protons from the mass number:

$$\text{Neutrons} = \text{Mass Number} - \text{Atomic Number}$$

This works because the atomic number is the number of protons.

5. Looking at the periodic table

On the periodic table, each element has an atomic number. That number tells you how many protons the element has.

For example:

• Hydrogen has atomic number 1.
• Helium has atomic number 2.
• Carbon has atomic number 6.
• Oxygen has atomic number 8.
• Sodium has atomic number 11.

If you know the atomic number, you know the element.

6. Worked Examples

Example 1: Find the atomic number

An atom has 8 protons. What is its atomic number?

Step 1: Remember that atomic number = number of protons.

Step 2: The atom has 8 protons.

Answer: The atomic number is 8.

Example 2: Find the mass number

An atom has 11 protons and 12 neutrons. What is its mass number?

Step 1: Use the formula

$$\text{Mass Number} = \text{Protons} + \text{Neutrons}$$

Step 2: Substitute the numbers.

$$11 + 12 = 23$$

Answer: The mass number is 23.

Example 3: Find the number of neutrons

An atom has atomic number 17 and mass number 35. How many neutrons does it have?

Step 1: Atomic number = protons, so the atom has 17 protons.

Step 2: Use the formula

$$\text{Neutrons} = \text{Mass Number} - \text{Atomic Number}$$

Step 3: Substitute the numbers.

$$35 - 17 = 18$$

Answer: The atom has 18 neutrons.

Example 4: Identify the element and its mass number

An atom has 6 protons and 8 neutrons.

1. What is its atomic number?
2. What is its mass number?
3. Which element is it?

Step 1: Atomic number = protons = 6.

Step 2: Mass number = protons + neutrons.

$$6 + 8 = 14$$

Step 3: The element with atomic number 6 is carbon.

Answers:

• Atomic number: 6
• Mass number: 14
• Element: carbon

7. Helpful ways to remember

• Atomic number = protons only
• Mass number = protons + neutrons
• Neutrons = mass number - atomic number

You can also remember it this way:

• Atomic number tells the atom's identity.
• Mass number tells the total particles in the nucleus.

8. Common mistakes to avoid

• Do not confuse atomic number with mass number.
• Do not include electrons when finding mass number.
• When finding neutrons, make sure you subtract correctly:

$$\text{Neutrons} = \text{Mass Number} - \text{Atomic Number}$$

9. Quick practice ideas

Try these on your own:

• An atom has 3 protons. What is its atomic number?
• An atom has 9 protons and 10 neutrons. What is its mass number?
• An atom has atomic number 12 and mass number 24. How many neutrons does it have?

10. Summary

The atomic number is the number of protons in an atom. It tells you which element the atom is.

The mass number is the total number of protons and neutrons in the nucleus. To find it, add protons and neutrons together.

If you know the mass number and atomic number, you can find neutrons by subtracting:

$$\text{Neutrons} = \text{Mass Number} - \text{Atomic Number}$$

When you understand these two numbers, you can describe atoms more clearly and identify elements correctly.

Atomic Structure Foundations helps us understand what all matter is made of. Matter is anything that takes up space and has mass, like water, air, rocks, and your body. Scientists have learned that all matter is made of tiny building blocks called atoms.

Atoms are very, very small. You cannot see one with just your eyes. But even though atoms are tiny, they are important because everything around us is made from them.

Each atom has smaller parts inside it. These parts are called protons, neutrons, and electrons. Learning where these parts are and what they do helps us understand how atoms work.

Main Idea: An atom has a center called the nucleus. The nucleus holds protons and neutrons. Electrons move around the outside of the nucleus.

Let’s look at the three main parts of an atom.

• Protons are found in the nucleus, in the center of the atom.
• Neutrons are also found in the nucleus.
• Electrons are found outside the nucleus and move around it.

It can help to picture an atom like a tiny center with even tinier pieces. The center is the nucleus. Around that center, electrons move in the space outside.

Protons are very important because the number of protons tells us what kind of atom it is. This number is called the atomic number.

If an atom has 1 proton, it is hydrogen. If an atom has 6 protons, it is carbon. If an atom has 8 protons, it is oxygen. So, the number of protons is like the atom’s name tag.

Atomic number = number of protons

We can write that as:

$$\text{Atomic number} = \text{number of protons}$$

This is one of the most important facts about atoms. No matter what else changes, if the number of protons changes, the atom becomes a different element.

An element is a pure kind of matter made of only one kind of atom. Gold is an element. Oxygen is an element. Helium is an element. Each element has its own number of protons.

Neutrons are also in the nucleus. Neutrons help make up the center of the atom. They do not change what element the atom is. The number of protons is still what decides the element.

Electrons move around the nucleus. They are part of the atom too, but they are not in the center. When we draw atoms at this level, we often show electrons as dots or small circles around the nucleus.

Here is a simple way to remember the parts:

• Protons: in the nucleus
• Neutrons: in the nucleus
• Electrons: outside the nucleus

You can also remember it this way: the center holds protons and neutrons, while electrons stay outside the center.

Atoms of different elements are different because they have different numbers of protons. This means every element has its own atomic number.

Here are a few examples:

• Hydrogen: 1 proton, atomic number 1
• Helium: 2 protons, atomic number 2
• Carbon: 6 protons, atomic number 6
• Oxygen: 8 protons, atomic number 8

If you know the number of protons, you can find the atomic number. If you know the atomic number, you know the number of protons.

Worked Example 1

A certain atom has 3 protons. What is its atomic number?

Step 1: Remember the rule:

$$\text{Atomic number} = \text{number of protons}$$

Step 2: Put in the number of protons, which is 3.

$$\text{Atomic number} = 3$$

Answer: The atomic number is 3.

Worked Example 2

An atom has 8 protons, 8 neutrons, and 8 electrons. What is its atomic number, and which part is in the center?

Step 1: The atomic number matches the number of protons.

$$\text{Atomic number} = 8$$

Step 2: Think about the center of the atom. The center is called the nucleus.

Step 3: The nucleus contains protons and neutrons.

Answer: The atomic number is 8, and the center contains protons and neutrons.

Worked Example 3

Which atom is a different element: Atom A has 5 protons. Atom B has 6 protons.

Step 1: Look at the number of protons in each atom.

• Atom A: 5 protons
• Atom B: 6 protons

Step 2: Different numbers of protons mean different elements.

Answer: Atom A and Atom B are different elements because they have different numbers of protons.

Worked Example 4

An atom has 2 protons. Another atom also has 2 protons. Are they the same element or different elements?

Step 1: Compare the number of protons.

Both atoms have 2 protons.

Step 2: The number of protons tells the element.

Answer: They are the same element because they have the same number of protons.

Let’s organize the important ideas in a simple chart.

• Nucleus: the center of the atom
• Protons: in the nucleus; tell the element
• Neutrons: in the nucleus
• Electrons: move around outside the nucleus
• Atomic number: the number of protons

Here is a simple comparison:

1. If the number of protons changes, the element changes.
2. If you know the atomic number, you know the number of protons.
3. Protons and neutrons are in the center.
4. Electrons are outside the center.

Sometimes students mix up where the parts go. A good way to check yourself is to ask:

• Is it in the center? Then it is a proton or neutron.
• Is it outside the center? Then it is an electron.

Another thing to remember is that atoms are the tiny parts that build all matter. When many atoms join together, they make the materials we see and use every day.

Even though atoms are too small to see, scientists use models and drawings to understand them. A model is a simple picture or idea that helps us learn about something very small or very hard to see.

In a model of an atom, you might see:

• a small center labeled nucleus
• protons and neutrons inside the nucleus
• electrons shown around the outside

This model is useful because it helps us remember where the parts are.

Let’s Review

• All matter is made of atoms.
• Atoms have three main parts: protons, neutrons, and electrons.
• Protons and neutrons are in the nucleus.
• Electrons move around outside the nucleus.
• The number of protons is the atomic number.
• The atomic number tells what element the atom is.

Brief Summary

Atoms are tiny building blocks of matter. Inside each atom is a nucleus that contains protons and neutrons. Electrons move around outside the nucleus. The number of protons is called the atomic number, and that number tells which element the atom is.

Isotopes and Average Atomic Mass

Everything around us is made of atoms. Atoms are tiny building blocks of matter. Even though atoms are very small, scientists have learned a lot about how they are put together.

To understand isotopes and average atomic mass, we first need to review the parts of an atom.

An atom has three main parts:

• Protons: positively charged particles in the nucleus
• Neutrons: particles with no charge in the nucleus
• Electrons: negatively charged particles moving around the nucleus

The nucleus is the center of the atom. It contains protons and neutrons.

The number of protons tells you what element the atom is. For example, every carbon atom has 6 protons. Every oxygen atom has 8 protons. If the number of protons changes, the element changes too.

However, atoms of the same element do not always have the same number of neutrons. When atoms of the same element have different numbers of neutrons, they are called isotopes.

Key idea: Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Because isotopes have different numbers of neutrons, they have different masses. This is why one element can have atoms with slightly different masses.

Example: Carbon always has 6 protons. But carbon can have different numbers of neutrons:

• Carbon-12 has 6 protons and 6 neutrons
• Carbon-13 has 6 protons and 7 neutrons
• Carbon-14 has 6 protons and 8 neutrons

These are all carbon atoms because they all have 6 protons. They are different isotopes because they have different numbers of neutrons.

The number after the element name, such as 12 in carbon-12, is called the mass number.

The mass number is found by adding the number of protons and neutrons:

$$\text{mass number} = \text{protons} + \text{neutrons}$$

So for carbon-13:

$$13 = 6 + 7$$

Now let’s think about a sample of an element found in nature. Most elements are not made of just one isotope. Instead, they are usually a mixture of isotopes.

This means that if you collect many atoms of an element, some may be one isotope and some may be another isotope.

Because of this mixture, the mass shown on the periodic table is usually not a whole number. It is an average atomic mass.

Average atomic mass is the average mass of all the naturally occurring isotopes of an element.

But this is not just a simple average. It is a weighted average. That means isotopes that are more common affect the average more than isotopes that are rare.

Here is the main idea:

• If one isotope is very common, the average atomic mass will be close to that isotope’s mass.
• If two isotopes occur in equal amounts, the average will be halfway between their masses.

To calculate average atomic mass, scientists multiply each isotope’s mass by its percent abundance written as a decimal. Then they add the results.

$$\text{average atomic mass} = (\text{isotope 1 mass} \times \text{decimal abundance}) + (\text{isotope 2 mass} \times \text{decimal abundance}) + \cdots$$

Important: Before multiplying, change each percent into a decimal.

• \(50\% = 0.50\)

• \(25\% = 0.25\)

• \(80\% = 0.80\)

Also, the total percent of all isotopes should add up to \(100\%\), or \(1.00\) as a decimal.

Worked Example 1: Identifying isotopes

An element has atoms with 8 protons. One atom has 8 neutrons. Another atom has 10 neutrons.

Are these the same element? Are they isotopes?

Step 1: Look at the number of protons.

Both atoms have 8 protons, so they are the same element.

Step 2: Look at the number of neutrons.

One has 8 neutrons and the other has 10 neutrons, so they have different numbers of neutrons.

Answer: Yes, they are isotopes of the same element.

Worked Example 2: Finding a mass number

A chlorine atom has 17 protons and 18 neutrons. What is its mass number?

Use the formula:

$$\text{mass number} = \text{protons} + \text{neutrons}$$

Substitute the numbers:

$$17 + 18 = 35$$

Answer: The mass number is 35. This isotope is chlorine-35.

Worked Example 3: Average atomic mass with two isotopes

Imagine an element has two isotopes:

• Isotope A: mass 10, abundance \(20\%\)
• Isotope B: mass 11, abundance \(80\%\)

Find the average atomic mass.

Step 1: Change percents to decimals.

• \(20\% = 0.20\)
• \(80\% = 0.80\)

Step 2: Multiply each mass by its decimal abundance.

$$10 \times 0.20 = 2.0$$

$$11 \times 0.80 = 8.8$$

Step 3: Add the results.

$$2.0 + 8.8 = 10.8$$

Answer: The average atomic mass is 10.8.

Notice that the answer is closer to 11 than to 10. That makes sense because the isotope with mass 11 is much more common.

Worked Example 4: Average atomic mass with three isotopes

An element has three isotopes:

• Mass 24, abundance \(79\%\)
• Mass 25, abundance \(10\%\)
• Mass 26, abundance \(11\%\)

Find the average atomic mass.

Step 1: Change each percent to a decimal.

• \(79\% = 0.79\)
• \(10\% = 0.10\)
• \(11\% = 0.11\)

Step 2: Multiply each isotope mass by its decimal abundance.

$$24 \times 0.79 = 18.96$$

$$25 \times 0.10 = 2.50$$

$$26 \times 0.11 = 2.86$$

Step 3: Add all the products.

$$18.96 + 2.50 + 2.86 = 24.32$$

Answer: The average atomic mass is 24.32.

Again, the answer is closest to 24 because the isotope with mass 24 is the most common.

How to remember the difference between these terms

• Atomic number: number of protons
• Mass number: protons + neutrons for one atom
• Isotope: same element, different neutrons
• Average atomic mass: weighted average of all the isotopes of an element

Common mistakes to avoid

• Do not confuse mass number with average atomic mass. Mass number is for one isotope. Average atomic mass is for a mixture of isotopes.
• Do not forget to change percentages to decimals before multiplying.
• Do not use the number of neutrons to identify the element. The number of protons identifies the element.
• Do not expect average atomic mass to always be a whole number. It is often a decimal because it is an average.

Why this matters

Scientists use isotopes to learn about materials, living things, and even the age of objects. But at your level, the most important idea is this: one element can come in slightly different forms, and the average atomic mass tells us the average mass of the atoms in nature.

Quick check for yourself

1. What is the one particle that must stay the same for atoms to be the same element?
2. If two atoms have the same number of protons but different numbers of neutrons, what are they called?
3. What do you add together to find mass number?
4. Why is average atomic mass often a decimal instead of a whole number?

Brief Summary

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Different isotopes have different masses because neutrons add mass. Since elements in nature are usually mixtures of isotopes, the periodic table shows an average atomic mass, which is a weighted average based on how common each isotope is.

Organization of the Periodic Table

The periodic table is a chart that organizes all known elements in a useful way. It helps scientists understand the properties of elements and predict how they may behave.

An element is a pure substance made of only one kind of atom. Each element has its own place on the periodic table.

The most important rule of the periodic table is this: elements are arranged in order of increasing atomic number. The atomic number tells how many protons are in the nucleus of an atom.

For example, hydrogen has atomic number 1, so it is first. Helium has atomic number 2, so it comes next. Lithium has atomic number 3, and so on. This pattern continues across the entire table.

You can think of the periodic table like a map. The position of an element gives clues about its structure and its properties.

Main Parts of the Periodic Table

The periodic table is organized into rows and columns.

• Rows are called periods.
• Columns are called groups or families.

Each element square usually includes some basic information, such as:

• the element name
• the chemical symbol
• the atomic number
• sometimes the atomic mass

Atomic Number and Order

The atomic number is what decides where an element goes on the table. As you move from left to right across a row, the atomic number increases by 1 each time.

For example, a simple number pattern at the beginning of the table is:

Hydrogen: \(1\), Helium: \(2\), Lithium: \(3\), Beryllium: \(4\)

This means the order is based on the number of protons, not on size, color, or whether the element is solid, liquid, or gas.

Periods: Horizontal Rows

A period is a horizontal row on the periodic table. Elements in the same period are on the same level across the chart.

As you move across a period from left to right:

• the atomic number increases
• the properties of the elements gradually change

For example, one period may begin with elements that are very reactive metals and end with elements that are less reactive gases.

This shows that the periodic table is not random. The arrangement lets us see patterns in element properties.

Groups: Vertical Columns

A group, also called a family, is a vertical column on the periodic table. Elements in the same group often have similar properties.

That means if two elements are in the same column, they may react in similar ways or share similar traits.

For example:

• elements in one group may all be shiny metals
• elements in another group may all be gases that do not react much

This is one reason the periodic table is so helpful. If scientists know the group of an element, they can make good predictions about its behavior.

Why Elements in a Group Are Similar

Elements in the same group have atoms with similar outer parts, which leads to similar chemical behavior. In 7th Grade, it is enough to remember this simple idea: same group often means similar properties.

For example, elements in the same family may:

• react with other elements in similar ways
• form similar kinds of compounds
• share physical traits, such as being metals or gases

Metals, Nonmetals, and Metalloids

The periodic table also helps classify elements into large categories.

• Metals are usually shiny, good conductors of heat and electricity, and bend without breaking easily.
• Nonmetals are usually dull and are not good conductors of heat and electricity.
• Metalloids have properties of both metals and nonmetals.

Most metals are found on the left side and center of the periodic table. Most nonmetals are on the right side. Metalloids are found along a stair-step line between them.

This placement makes it easier to compare elements quickly.

Important Families on the Periodic Table

You do not need to memorize every group, but some families are especially important.

• Alkali metals: found in the first column on the left, except for hydrogen. These are very reactive metals.
• Alkaline earth metals: found in the second column. These are reactive metals too, but usually less reactive than alkali metals.
• Halogens: found near the right side of the table. These are very reactive nonmetals.
• Noble gases: found in the far right column. These elements are very unreactive.

These family names help scientists talk about groups of elements that behave in similar ways.

How to Read an Element Square

Each box on the periodic table gives key information. For example, an element square might show:

• Symbol: a short abbreviation, such as O for oxygen
• Name: oxygen
• Atomic number: 8
• Atomic mass: a number that tells about the average mass of the atom

The most important number for organizing the table is the atomic number.

Patterns on the Table

The word periodic means repeating in a pattern. The periodic table has repeating patterns of properties.

For example, if one group contains reactive metals, other elements in that same group are usually reactive in similar ways. If one group contains gases that barely react, the others in that group are usually similar too.

This repeating pattern is why the table is called the periodic table.

Worked Example 1: Finding the Next Element by Atomic Number

Question: If an element has atomic number 11, what atomic number comes right after it on the periodic table?

Step 1: Remember that elements are arranged in increasing atomic number.

Step 2: Count up by 1.

$$11 + 1 = 12$$

Answer: The next element has atomic number 12.

Worked Example 2: Identifying a Group and Period

Question: Two elements are in the same vertical column. Are they in the same group or the same period?

Step 1: Remember that vertical columns are called groups.

Step 2: Horizontal rows are called periods.

Answer: If two elements are in the same vertical column, they are in the same group.

Worked Example 3: Predicting Similar Properties

Question: Element A and Element B are in the same group. What can you predict about them?

Step 1: Recall that elements in the same group often have similar properties.

Step 2: Think about what “similar properties” means. They may react in similar ways or belong to the same category, such as metals or nonmetals.

Answer: You can predict that Element A and Element B will have similar chemical properties.

Worked Example 4: Classifying Position on the Table

Question: An element is located on the far right side of the periodic table in a group of very unreactive gases. Which family is it most likely in?

Step 1: Look for the clue “far right side.”

Step 2: Look for the clue “very unreactive gases.”

Step 3: Match that description to a family.

Answer: It is most likely in the noble gases family.

Common Mistakes to Avoid

• Do not confuse groups and periods. Groups go up and down. Periods go left and right.
• Do not think elements are arranged by atomic mass. The modern periodic table is arranged by atomic number.
• Do not assume all elements are the same just because they are close together. The most similar elements are usually in the same group.
• Do not forget that metals and nonmetals are usually found in different areas of the table.

Why This Matters

The periodic table is one of the most important tools in science. It helps us organize information about matter and understand how different elements compare to one another.

By learning how the table is arranged, you can look at an element’s position and quickly learn useful information about it.

Lesson Summary

The periodic table arranges elements by increasing atomic number. Its horizontal rows are called periods, and its vertical columns are called groups or families.

Elements in the same group often have similar properties. The table also helps classify elements as metals, nonmetals, or metalloids.

When you understand the organization of the periodic table, you can use an element’s location to predict important facts about it.

Metals, Nonmetals, and Metalloids are three big groups of elements on the periodic table. Learning these groups helps us predict how different elements behave. For example, we can often tell whether an element will conduct electricity, bend easily, or react with other substances just by knowing which group it belongs to.

Everything around us is made of matter, and matter is made of tiny particles called atoms. An element is a pure substance made of only one kind of atom. Scientists organize elements on the periodic table, and their locations help show patterns in their properties.

One important pattern is that most elements can be grouped as metals, nonmetals, or metalloids. These groups have different physical and chemical properties. Physical properties are things we can observe, like color, shininess, and whether something can be bent. Chemical properties describe how a substance reacts with other substances.

Where are these groups on the periodic table?

• Metals are found on the left side and in the center of the periodic table.
• Nonmetals are found on the right side of the periodic table.
• Metalloids are found along the zigzag or staircase line between metals and nonmetals.

This arrangement is useful because elements in the same broad group often act in similar ways.

1. Metals

Metals are the largest group of elements. Many everyday objects are made from metals, such as aluminum cans, copper wires, and iron tools.

Most metals share these properties:

• Shiny when polished
• Good conductors of heat and electricity
• Malleable, which means they can be hammered or pressed into shapes
• Ductile, which means they can be pulled into wires
• Usually solid at room temperature

Because metals conduct electricity well, they are often used in wires and electronics. Because they are malleable, they are useful for making pans, car parts, and building materials.

Examples of metals include:

• Iron 28Fe: used in construction and tools
• Copper 29Cu: used in electrical wires
• Aluminum 13Al: used in cans and airplanes
• Gold 79Au: used in jewelry and electronics

Many metals are also reactive, which means they can combine with other substances. Some metals react strongly with water or oxygen, while others react more slowly. For example, iron can react with oxygen and water to form rust.

2. Nonmetals

Nonmetals have properties that are very different from metals. Many gases in the air are nonmetals, and some solids like sulfur are nonmetals too.

Most nonmetals share these properties:

• Often dull, not shiny
• Poor conductors of heat and electricity
• If solid, usually brittle, meaning they break easily instead of bending
• Many are gases at room temperature

Because nonmetals do not conduct electricity well, they are often used as insulators. An insulator is a material that does not let electric current flow easily. For example, plastic around a wire is made from nonmetal elements in compounds that help keep people safe.

Examples of nonmetals include:

• Oxygen 8O: important for breathing
• Carbon 6C: found in living things and many materials
• Sulfur 16S: a yellow solid
• Nitrogen 7N: makes up much of Earths atmosphere

Some nonmetals are very reactive. Oxygen, for example, reacts with many other elements. Other nonmetals, such as some gases, are much less reactive.

3. Metalloids

Metalloids are elements with properties of both metals and nonmetals. They are found along the staircase line on the periodic table. Metalloids are useful because they can behave in special ways under different conditions.

Common properties of metalloids include:

• They may look shiny like metals or dull like nonmetals
• They are usually brittle, not malleable
• They conduct electricity better than nonmetals but not as well as metals

A material that only conducts electricity under some conditions is called a semiconductor. This makes metalloids very important in electronics such as computers, phones, and solar panels.

Examples of metalloids include:

• Silicon 14Si
• Boron 5B
• Germanium 32Ge

Comparing the three groups

It helps to compare the groups side by side.

• Metals: shiny, bendable, good conductors
• Nonmetals: dull, brittle if solid, poor conductors
• Metalloids: in between, often brittle, semiconductors

We can also compare them by important properties:

• Conductivity: Metals are highest, nonmetals are lowest, metalloids are in the middle.
• Malleability: Metals are usually malleable, while nonmetals and metalloids are usually not.
• Reactivity: Reactivity varies, but knowing the group gives us clues about how an element may behave.

How these categories help us predict properties

Suppose you find an unknown element. If it is shiny, can be made into wire, and carries electricity well, it is probably a metal. If it is dull, breaks easily, and does not carry electricity well, it is probably a nonmetal. If it has a mix of these properties, especially semiconducting behavior, it is likely a metalloid.

Scientists use these patterns to decide how materials should be used. A good electrical conductor is helpful for power lines, so metals are used. A poor conductor is useful for safety coverings, so nonmetals are used in insulating materials. Materials that control electrical flow are useful in technology, so metalloids are used in computer chips.

Worked Example 1: Identifying a metal

An element is shiny, can be hammered into a flat sheet, and conducts electricity very well. What group does it belong to?

Step 1: Look at the clues. The element is shiny, malleable, and a good conductor.

Step 2: Match the clues to a group. These are classic properties of metals.

Answer: The element is most likely a metal.

Worked Example 2: Identifying a nonmetal

A solid element is dull and breaks apart when hit with a hammer. It does not conduct electricity well. What group does it belong to?

Step 1: Notice the properties: dull, brittle, poor conductor.

Step 2: Compare with the three groups. These properties fit a nonmetal.

Answer: The element is most likely a nonmetal.

Worked Example 3: Identifying a metalloid

An element is brittle like a nonmetal, but it can conduct some electricity and is used in computer chips. What group does it belong to?

Step 1: The element has mixed properties. It is brittle, but it also conducts some electricity.

Step 2: Elements with both metal and nonmetal traits are metalloids.

Answer: The element is most likely a metalloid.

Worked Example 4: Predicting use from properties

Which type of element would be best for electrical wiring: a metal, a nonmetal, or a metalloid?

Step 1: Think about the job. Electrical wires need to let electricity flow easily.

Step 2: Metals are the best conductors of electricity.

Step 3: Metals are also ductile, so they can be pulled into long wires.

Answer: A metal would be the best choice for electrical wiring.

Important idea: not every element matches perfectly

In science, there can be exceptions. For example, not all metals have exactly the same level of reactivity, and not all nonmetals are gases. The groups are still useful because they help us see the general patterns.

Think of the groups as a guide. They do not tell us everything about an element, but they help us make strong predictions.

Tips for remembering the groups

• Metals = left and center, shiny, bendable, conductors
• Nonmetals = right side, dull, brittle or gases, poor conductors
• Metalloids = staircase line, mixed properties, semiconductors

Brief Summary

Elements are grouped as metals, nonmetals, and metalloids based on their properties. Metals are usually shiny, malleable, and good conductors. Nonmetals are usually dull, brittle if solid, and poor conductors. Metalloids have mixed properties and are useful as semiconductors. By learning these groups, we can predict how an element may conduct electricity, bend, or react.