Properties and States of Matter

Matter and Mass

Everything around you is made of matter. The air in a balloon, the water in a bottle, your pencil, your desk, and even your body are all matter.

To understand matter, it helps to ask two questions: What is it? and How much of it is there? This lesson will help you answer both questions by learning about matter, mass, and how mass is different from weight.

What is matter?

Matter is anything that has mass and takes up space. Taking up space means it has volume.

So, for something to be matter, it must do two things:

• have mass
• take up space

Many things are matter, including:

• rocks
• milk
• air
• books
• people

Some things are not matter because they do not have mass and do not take up space. Examples include:

• light
• sound
• heat

What is mass?

Mass is the amount of matter in an object. If one object has more matter in it than another object, it has more mass.

Mass does not depend on where an object is. A basketball has the same mass in your classroom, on top of a mountain, or on the Moon.

Mass is usually measured with a balance. Common metric units for mass are:

• grams (g) for smaller objects
• kilograms (kg) for larger objects

Remember:

• $$1 \text{ kg} = 1000 \text{ g}$$

What is weight?

Weight is different from mass. Weight is the pull of gravity on an object.

Gravity pulls objects toward the ground. On Earth, gravity gives objects weight. If gravity changes, weight changes too.

For example, an astronaut on the Moon would weigh less than on Earth because the Moon has less gravity. But the astronaut's mass stays the same because the amount of matter in the astronaut does not change.

Mass and weight are not the same

People sometimes use the words mass and weight as if they mean the same thing, but in science they are different.

• Mass = amount of matter
• Weight = pull of gravity on that matter

Here is a simple comparison:

• Mass stays the same in different places.
• Weight can change if gravity changes.
• Mass is measured in grams or kilograms.
• Weight is measured with a spring scale.

How can you tell if something is matter?

You can ask:

1. Does it have mass?
2. Does it take up space?

If the answer to both is yes, it is matter.

For example, air may seem invisible, but it is matter. Air fills a tire or balloon, which shows that it takes up space. Air also has mass.

States of matter and mass

Matter can exist in different states, such as solid, liquid, and gas.

• A solid has its own shape, like a rock or a book.
• A liquid flows and takes the shape of its container, like water.
• A gas spreads out to fill its container, like air.

No matter the state, matter still has mass. Ice, liquid water, and water vapor are all forms of matter. They may look different, but each one has mass and takes up space.

Worked Example 1: Is it matter?

Question: Which of these are matter: a soccer ball, sunlight, and air?

Step 1: Ask if each one has mass and takes up space.

• A soccer ball has mass and takes up space, so it is matter.
• Sunlight does not take up space like an object and is not made of matter, so it is not matter.
• Air has mass and takes up space, so it is matter.

Answer: The soccer ball and air are matter. Sunlight is not matter.

Worked Example 2: Comparing mass

Question: A science book has a mass of 500 g. A notebook has a mass of 200 g. Which has more mass, and how much more?

Step 1: Compare the numbers.

500 g is greater than 200 g, so the science book has more mass.

Step 2: Find the difference.

$$500 - 200 = 300$$

Answer: The science book has more mass. It has 300 g more mass than the notebook.

Worked Example 3: Converting units

Question: A watermelon has a mass of 3 kg. How many grams is that?

Step 1: Use the fact that

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

Step 2: Multiply by 3.

$$3 \times 1000 = 3000$$

Answer: The watermelon has a mass of 3000 g.

Worked Example 4: Mass or weight?

Question: A student says, “If I go to the Moon, my mass will be smaller.” Is this correct?

Step 1: Remember the definitions.

• Mass is the amount of matter.
• Weight is the pull of gravity.

Step 2: Think about what changes on the Moon.

The amount of matter in the student does not change, but the pull of gravity does.

Answer: The statement is not correct. The student's mass stays the same, but the student's weight becomes smaller on the Moon.

Common mistakes to avoid

• Do not say mass and weight mean the same thing in science.
• Do not forget that gases like air are matter.
• Do not confuse size with mass. A larger object does not always have more mass than a smaller object.

For example, a large empty box may look bigger than a small rock, but the rock may have more mass.

Why this matters in science

Scientists use mass to describe how much matter is in a sample. This helps them compare objects, measure materials, and study changes in matter.

Understanding the difference between mass and weight also helps explain what happens in different places, such as on Earth and on the Moon.

Lesson Summary

Matter is anything that has mass and takes up space. Mass is the amount of matter in an object, and it is usually measured in grams or kilograms. Weight is the pull of gravity on an object. Mass stays the same from place to place, but weight can change if gravity changes.

Kinetic Molecular Theory helps us explain how matter behaves by thinking about tiny particles that are always moving.

Even though we cannot usually see these particles, they make up all matter. Matter includes solids, liquids, and gases. The way the particles move and how close they are to each other help explain the properties of each state of matter.

This lesson will show how the motion of tiny particles explains things we can observe, like why a balloon expands, why perfume spreads through a room, and why ice keeps its shape while water flows.

What is Kinetic Molecular Theory?

The word kinetic means motion. The word molecular refers to the tiny particles that make up matter. So, Kinetic Molecular Theory is the idea that all matter is made of tiny particles that are always moving.

These particles may be atoms, molecules, or other tiny pieces of matter. In 6th Grade science, it is enough to remember that they are too small to see and are always in motion.

Main ideas of Kinetic Molecular Theory

• All matter is made of tiny particles.
• The particles are always moving.
• There are spaces between particles.
• Particles push and bump into each other and into their container.
• When matter is heated, particles move faster.
• When matter is cooled, particles move slower.

These ideas help explain many physical properties of matter.

Particles in solids, liquids, and gases

The particles in each state of matter move in different ways. This is why solids, liquids, and gases have different properties.

1. Solids

In a solid, particles are packed very close together. They do not move from place to place, but they do vibrate, or shake in place.

• Solids have a definite shape.
• Solids have a definite volume.
• The particles stay in fixed positions, so the solid keeps its shape.

For example, an ice cube keeps its shape because its particles are close together and only vibrate in place.

2. Liquids

In a liquid, particles are still close together, but they can slide past each other.

• Liquids have a definite volume.
• Liquids do not have a definite shape.
• Liquids take the shape of their container.

For example, water in a cup takes the shape of the cup because its particles can move around each other.

3. Gases

In a gas, particles are much farther apart and move freely in many directions.

• Gases do not have a definite shape.
• Gases do not have a definite volume.
• Gases spread out to fill their container.

For example, air in a balloon spreads out and fills the whole balloon because gas particles move freely and are far apart.

How temperature affects particle motion

Temperature tells us about how fast particles are moving. When particles move faster, they have more kinetic energy. In simple terms, they have more energy of motion.

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

This idea helps explain many everyday changes:

• Hot air expands because the gas particles move faster and spread out more.
• Cold air takes up less space because the particles move slower.
• Warm water particles move faster than cold water particles.
• Ice melts when particles gain enough energy to move more freely.

Particle motion and changes of state

A change of state happens when matter changes from one form to another, such as from solid to liquid or liquid to gas.

Kinetic Molecular Theory explains these changes by the motion of particles.

• Melting: A solid is heated. Its particles vibrate faster until they can move past each other. The solid becomes a liquid.
• Freezing: A liquid is cooled. Its particles slow down and stay in place. The liquid becomes a solid.
• Evaporation/Boiling: A liquid is heated. Its particles move fast enough to escape into the air as a gas.
• Condensation: A gas is cooled. Its particles slow down and come closer together to form a liquid.

So, changes of state happen because particles gain or lose energy and change how they move.

Why gases create pressure

Gas particles move quickly and bump into the walls of their container. These collisions create pressure.

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

That is why heating a sealed container can be dangerous. The particles inside move faster and create more pressure.

How Kinetic Molecular Theory explains everyday observations

Here are some things you may notice in daily life and how this theory explains them:

• Perfume spreads across a room: Gas particles move in all directions and mix with air.
• Food coloring spreads in water: Liquid particles are moving, so the color slowly spreads out.
• A basketball feels flatter in the cold: The gas particles inside move slower and push less.
• A metal lid loosens under warm water: Heating causes particles to move more and spread slightly apart.

Worked Example 1: Identifying the state of matter

Question: A substance has a definite volume, but it takes the shape of its container. What state of matter is it?

Step 1: Think about the properties.

• Definite volume
• No definite shape

Step 2: Match those properties to a state of matter.

Solids have definite shape and volume. Gases have neither definite shape nor definite volume. Liquids have definite volume but take the shape of their container.

Answer: The substance is a liquid.

Worked Example 2: Explaining heating

Question: What happens to the particles in water when the water is heated?

Step 1: Recall the rule from Kinetic Molecular Theory.

When matter is heated, particles move faster.

Step 2: Apply it to water.

The particles in the water gain energy and move faster. If enough heat is added, some particles can escape as gas.

Answer: The water particles move faster when heated.

Worked Example 3: Explaining a real-life observation

Question: Why does an unopened chip bag puff up when taken to a higher place, like a mountain?

Step 1: Remember that gas particles are always moving and pushing on the bag.

Step 2: At higher places, the air outside the bag pushes less than it did before.

Step 3: The gas inside the bag spreads out more, making the bag look puffier.

Answer: The bag puffs up because the gas particles inside spread out more and push on the bag.

Worked Example 4: Comparing particle motion

Question: In which state of matter do particles move most freely: solid, liquid, or gas?

Step 1: Compare particle motion in each state.

• In solids, particles vibrate in place.
• In liquids, particles slide past each other.
• In gases, particles move freely in all directions.

Step 2: Choose the state with the greatest freedom of movement.

Answer: Particles move most freely in a gas.

Common mistakes to avoid

• Mistake: Thinking particles stop moving in a solid.
  Particles in solids still move. They vibrate in place.
• Mistake: Thinking empty space means nothing is there in gases.
  Gas particles are far apart, but the gas still fills the space.
• Mistake: Thinking heating always changes the kind of matter.
  Heating usually changes particle motion or state, not what the substance is.
• Mistake: Thinking liquids have no volume.
  Liquids do have volume. They just do not keep a fixed shape.

Quick check for understanding

1. What does the word kinetic mean?
2. Are particles in matter ever completely still?
3. How are particles arranged in a solid compared with a gas?
4. What usually happens to particle motion when matter is heated?
5. Why do gases fill the shape of any container?

Answers:

1. Kinetic means motion.
2. No. Particles are always moving.
3. In a solid, particles are close together. In a gas, particles are far apart.
4. Particles move faster.
5. Gas particles move freely in all directions and spread out.

Lesson Summary

Kinetic Molecular Theory says that all matter is made of tiny particles that are always moving. The speed of particle motion and the spacing between particles help explain the properties of solids, liquids, and gases.

In solids, particles are close together and vibrate in place. In liquids, particles are close together but can slide past one another. In gases, particles are far apart and move freely.

Heating makes particles move faster, and cooling makes them move slower. This helps explain changes of state, pressure in gases, and many everyday observations around us.

States of Matter tells us the different ways matter can be. Matter is anything that takes up space and has weight. A rock is matter. Water is matter. Air is matter too.

There are different states of matter. In this lesson, we will learn about solids, liquids, gases, and plasmas.

We can tell the states of matter apart by how they look, how they move, and whether they keep their shape.

1. Solids

A solid has its own shape. It stays the same shape unless someone changes it. A book is a solid. A chair is a solid. An ice cube is a solid.

The tiny pieces of a solid are packed closely together. They do not move around much. That is why solids keep their shape.

• Solids have a shape of their own.
• Solids take up space.
• Solids can be hard, soft, rough, or smooth.

2. Liquids

A liquid does not have its own shape. A liquid takes the shape of its container. If you pour water into a cup, it looks like the cup. If you pour it into a bowl, it looks like the bowl.

The tiny pieces of a liquid are close together, but they can slide past each other. That is why liquids can flow.

• Liquids do not keep one shape.
• Liquids take the shape of their container.
• Liquids can be poured.

Examples of liquids are water, milk, and juice.

3. Gases

A gas does not have its own shape. It also does not stay in one place like a liquid in a cup. A gas spreads out to fill the space around it.

The tiny pieces of a gas are far apart and move around quickly. That is why gases can spread out.

• Gases do not have their own shape.
• Gases spread out to fill space.
• Many gases cannot be seen.

Air is a gas. The air in a balloon fills the balloon. The gas takes the shape of the balloon.

4. Plasma

Plasma is another state of matter. It is not as common around us as solids, liquids, and gases, but it is important.

Plasma is made when gas gets very, very hot. The tiny pieces move with lots of energy. The Sun is made of plasma. Lightning is also plasma.

• Plasma is very hot.
• Plasma gives off light.
• The Sun and lightning are examples of plasma.

How Matter Can Change

Matter can change from one state to another when it gets warmer or cooler.

When a solid gets warmer, it can melt and become a liquid. Ice melts into water.

When a liquid gets cooler, it can freeze and become a solid. Water freezes into ice.

When a liquid gets warmer, it can become a gas. Water can turn into water vapor.

When a gas gets cooler, it can turn back into a liquid. You may see tiny drops of water on a cold glass.

Comparing the States of Matter

• Solid: keeps its own shape
• Liquid: takes the shape of its container
• Gas: spreads out and fills space
• Plasma: very hot matter that gives off light

Worked Example 1

Question: Is an ice cube a solid, liquid, gas, or plasma?

Step 1: Think about its shape. An ice cube keeps its own shape.

Step 2: Solids keep their own shape.

Answer: An ice cube is a solid.

Worked Example 2

Question: You pour milk from a carton into a glass. What state of matter is the milk?

Step 1: Think about what milk does. It can be poured.

Step 2: It takes the shape of the glass.

Answer: Milk is a liquid.

Worked Example 3

Question: What state of matter is the air inside a balloon?

Step 1: Air does not have its own shape.

Step 2: It fills the balloon.

Answer: Air is a gas.

Worked Example 4

Question: The Sun shines and is very, very hot. What state of matter is it?

Step 1: Plasma is very hot and gives off light.

Step 2: The Sun is very hot and gives off light.

Answer: The Sun is made of plasma.

Things to Remember

1. All matter takes up space.
2. A solid keeps its own shape.
3. A liquid can be poured and takes the shape of its container.
4. A gas spreads out and fills space.
5. Plasma is very hot matter, like the Sun and lightning.
6. Matter can change states when it gets warmer or cooler.

Brief Summary

Matter can be a solid, liquid, gas, or plasma. Solids keep their shape. Liquids take the shape of their container. Gases spread out to fill space. Plasma is very hot matter that gives off light. When matter heats up or cools down, it can change from one state to another.

States of Matter are 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, the water you drink, and the desk you sit at.

The main states of matter you will learn in 6th Grade are solid, liquid, gas, and plasma. These states are different because the particles in them are arranged differently, have different amounts of space between them, and move with different amounts of energy.

To understand states of matter, it helps to think about particles. Particles are tiny pieces of matter. You cannot usually see them, but they are always moving. How closely packed they are and how fast they move helps decide whether a substance is a solid, liquid, gas, or 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 certain amount of space.

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

• Shape: definite
• Volume: definite
• Particle arrangement: tightly packed
• Particle motion: vibrate in place
• Energy: lowest of the four states

Examples of solids include ice, a rock, a pencil, and a metal spoon.

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 are not locked in place. They can slide past one another. This is why liquids can flow.

• Shape: takes the shape of its container
• Volume: definite
• Particle arrangement: close together but not fixed
• Particle motion: slide past one another
• Energy: more than solids

Examples of liquids include water, juice, milk, and cooking oil.

3. Gases

A gas has no definite shape and no definite volume. A gas spreads out to fill all the space in its container.

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

• Shape: no definite shape
• Volume: no definite volume
• Particle arrangement: far apart
• Particle motion: move freely and quickly
• Energy: more than liquids

Examples of gases include oxygen, helium, water vapor, and the air in a balloon.

4. Plasma

Plasma is a state of matter that forms when gas gets so much energy that its particles change. Plasma is like an energized gas.

In plasma, particles move very fast, and some particles are broken into charged pieces. Plasma is less common in everyday life than solids, liquids, and gases, but it is very important in nature and technology.

• Shape: no definite shape
• Volume: no definite volume
• Particle arrangement: spread out
• Particle motion: very fast
• Energy: highest of the four states

Examples of plasma include lightning, the Sun, stars, and glowing neon signs.

Comparing the Four States

The four states of matter can be compared by looking at three big ideas: particle arrangement, spacing, and kinetic energy.

Kinetic energy means energy of motion. When particles have more kinetic energy, they move faster. In general:

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

As you move from solid to plasma, particles usually:

• move faster,
• have more energy, and
• spread farther apart.

Quick comparison:

• Solid: tightly packed particles, lowest energy
• Liquid: close particles that can flow
• Gas: far-apart particles moving quickly
• Plasma: very high-energy particles

Changes of State

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

Here are the main changes:

• Melting: solid to liquid
• Freezing: liquid to solid
• Evaporation/Boiling: liquid to gas
• Condensation: gas to liquid
• Sublimation: solid to gas
• Deposition: gas to solid

When energy is added, particles move faster. When energy is removed, particles move more slowly.

For example, when ice melts, heat is added. The particles gain energy and can move more freely, so the solid becomes a liquid. When water freezes, energy is removed. The particles slow down and become more fixed in place.

Worked Example 1: Identifying a Solid

Question: A wooden block keeps its shape when you move it from a desk to a shelf. What state of matter is it?

Step 1: Look at its shape. It keeps the same shape.

Step 2: Think about volume. It keeps the same size too.

Answer: It is a solid because solids have a definite shape and a definite volume.

Worked Example 2: Identifying a Liquid

Question: Orange juice is poured from a carton into a glass. It changes shape to fit the glass, but the amount of juice stays the same. What state of matter is it?

Step 1: It changes shape, so it does not have a definite shape.

Step 2: The amount stays the same, so it has a definite volume.

Answer: It is a liquid.

Worked Example 3: Comparing Gas and Plasma

Question: Both gas and plasma can spread out to fill a container. How are they different?

Step 1: Remember that gas particles move quickly and are far apart.

Step 2: Plasma has even more energy than gas.

Step 3: Plasma is made of very energized particles and is found in things like stars and lightning.

Answer: Plasma is different from gas because it has more energy and forms under very high-energy conditions.

Worked Example 4: Change of State

Question: Water in a pot is heated until it becomes steam. What change of state happened?

Step 1: Water starts as a liquid.

Step 2: Steam is water in the gas state.

Step 3: A change from liquid to gas is evaporation or boiling.

Answer: The change of state is boiling (liquid to gas).

Common Mistakes to Avoid

• Mistake 1: Thinking gases have no mass. Gases are still matter, so they have mass and take up space.
• Mistake 2: Thinking liquids have no volume. Liquids do have a definite volume; they just do not keep their own shape.
• Mistake 3: Thinking plasma and gas are exactly the same. Plasma is similar to gas, but it has much more energy.
• Mistake 4: Thinking particles in solids do not move at all. They do move, but they mainly vibrate in place.

Why This Matters

Understanding states of matter helps explain many things you see every day. Ice melting, puddles drying, steam rising from soup, and lightning in the sky all involve matter and changes in energy.

Scientists use this idea to study weather, cooking, materials, and even stars. Learning how particles behave helps us understand the world better.

Lesson Summary

Matter exists mainly as solids, liquids, gases, and plasmas. These states are different because of how their particles are arranged, how far apart the particles are, and how much kinetic energy the particles have.

Solids have tightly packed particles and keep their shape. Liquids have particles that stay close together but can flow. Gases have particles that are far apart and move freely. Plasma has the highest energy and is found in things like stars and lightning.

When energy is added or removed, matter can change from one state to another. Knowing these patterns helps you identify and compare the states of matter in the world around you.

Thermal Energy and Temperature

Have you ever touched a warm mug of cocoa and a cool glass of water? Both have energy, but they do not feel the same. In science, we use special words to talk about this: temperature and thermal energy.

These two ideas are connected, but they are not the same thing. Learning the difference helps us understand why some things feel hotter or colder, and why heat moves from one place to another.

Temperature tells how hot or cold something is. It is a measure of how fast the tiny particles in something are moving. When particles move faster, the temperature is higher. When particles move slower, the temperature is lower.

Thermal energy is the total energy of all the moving particles in an object. This means thermal energy depends on how fast the particles are moving and also how much matter there is.

So here is the big idea:

• Temperature = how fast particles are moving on average
• Thermal energy = the total energy of all the particles moving in the whole object

You can think of it like this: temperature is about the average, and thermal energy is about the total.

Imagine two cups of water:

• One small cup of hot water
• One large bowl of warm water

The small cup may have a higher temperature because it is hotter. But the large bowl may have more thermal energy because it has much more water.

This is why a bathtub of warm water has more thermal energy than a spoonful of hot water. The spoonful may have a higher temperature, but the bathtub has much more matter.

What causes thermal energy and temperature?

All matter is made of tiny particles. These particles are always moving, even when we cannot see them. In solids, the particles mostly wiggle in place. In liquids, they slide past each other. In gases, they move around freely.

When particles move faster, the object has a higher temperature. When there are many particles moving, the object can have a lot of thermal energy.

How do we measure temperature?

We measure temperature with a thermometer. A thermometer can show numbers in degrees, such as degrees Fahrenheit or degrees Celsius. A bigger number means a higher temperature.

Temperature is often written with a degree symbol, like \(20^\circ\) or \(75^\circ\).

How does thermal energy move?

Thermal energy naturally moves from a warmer object to a cooler object. This movement is called heat transfer.

For example, if you hold an ice cube in your hand, thermal energy moves from your warmer hand to the colder ice cube. Your hand may feel cooler, and the ice cube starts to melt.

The important idea is this: energy moves from warm to cool.

Cold is not something that travels. Cold is not a kind of energy. Instead, when something feels cold, it means it has less thermal energy than something warmer nearby.

So if you touch a cold metal spoon, the spoon is not sending “cold” into your hand. Instead, thermal energy is moving from your hand into the spoon. That is why your hand feels cold.

Main differences between temperature and thermal energy

• Temperature tells how hot or cold something is.
• Thermal energy tells how much total moving-particle energy an object has.
• Temperature depends on the average motion of particles.
• Thermal energy depends on both particle motion and the amount of matter.

Worked Example 1: Small hot cup and large warm bowl

A small cup of soup is very hot. A large pot of soup is warm, but not as hot as the cup.

Question: Which has the higher temperature?

Answer: The small cup has the higher temperature because it is hotter.

Question: Which might have more thermal energy?

Answer: The large pot might have more thermal energy because it has much more soup. Even though it is not as hot, it has more matter.

Worked Example 2: Two bowls of water

Two bowls have the same amount of water. Bowl A is at \(30^\circ\). Bowl B is at \(20^\circ\).

Question: Which bowl has the higher temperature?

Answer: Bowl A has the higher temperature because \(30^\circ\) is greater than \(20^\circ\).

Question: Which bowl has more thermal energy?

Answer: Bowl A also has more thermal energy in this case. The two bowls have the same amount of water, so the hotter one has more total moving-particle energy.

Worked Example 3: Why ice melts in your hand

You pick up an ice cube with your warm hand.

Question: Does cold move from the ice to your hand?

Answer: No. Cold does not move like a thing.

What really happens?

1. Your hand has a higher temperature than the ice.
2. Thermal energy moves from your hand to the ice.
3. The ice gains energy and begins to melt.
4. Your hand loses some energy, so it feels colder.

Worked Example 4: Same temperature, different thermal energy

A cup of water and a bucket of water are both at \(25^\circ\).

Question: Do they have the same temperature?

Answer: Yes. Both are at \(25^\circ\).

Question: Do they have the same thermal energy?

Answer: No. The bucket has more thermal energy because it has more water. There are more particles, so there is more total energy.

Helpful way to remember

• Temperature = how hot something is
• Thermal energy = how much total heat energy is in the whole object
• More matter can mean more thermal energy
• Hotter temperature means particles are moving faster

Things students sometimes mix up

• Mix-up: Bigger objects always have higher temperature.
  Truth: Bigger objects can have more thermal energy, but not always a higher temperature.
• Mix-up: Cold moves from one object to another.
  Truth: Thermal energy moves from warmer objects to cooler objects.
• Mix-up: Temperature and thermal energy mean the same thing.
  Truth: Temperature is about average particle motion. Thermal energy is about total energy in all the particles.

Summary

Temperature and thermal energy are related, but they are different. Temperature tells how hot or cold something is by showing how fast its particles move on average.

Thermal energy is the total energy of all the moving particles in an object. A large warm object can have more thermal energy than a small hot object.

Thermal energy moves from warmer places to cooler places. Cold is not a separate thing that flows. When something feels cold, it simply has less thermal energy than something warmer nearby.

Phase transitions are changes from one state of matter to another. Matter can be a solid, liquid, or gas. When matter changes state, the particles do not become a new substance. It is still the same matter, but its particles move differently and are arranged differently.

For example, ice, liquid water, and water vapor are all made of water. The only difference is the state they are in. A phase transition happens when matter gains energy or loses energy, usually in the form of heat.

To understand phase transitions, it helps to think about particles. In a solid, particles are packed closely together and mostly vibrate in place. In a liquid, particles are still close together, but they can slide past each other. In a gas, particles are spread far apart and move freely.

When particles gain energy, they usually move faster and spread out more. When particles lose energy, they slow down and move closer together. This change in particle motion helps explain all phase transitions.

Main idea:

• If matter absorbs energy, particles move faster.
• If matter releases energy, particles move slower.

There are six important phase transitions to know in 6th Grade Science.

1. Melting: solid 92 liquid
2. Freezing: liquid 92 solid
3. Vaporization: liquid 92 gas
4. Condensation: gas 92 liquid
5. Sublimation: solid 92 gas
6. Deposition: gas 92 solid

Lets look at each one more closely.

1. Melting

Melting happens when a solid changes into a liquid. This happens when the solid absorbs energy. As the particles gain energy, they vibrate faster and begin to break out of their fixed positions.

A common example is ice melting into liquid water. The ice absorbs heat from the air or from a warm surface. The water particles then move more freely, so the solid becomes a liquid.

2. Freezing

Freezing happens when a liquid changes into a solid. This happens when the liquid releases energy. As the particles lose energy, they slow down and move less. Eventually, they lock into place and form a solid.

Liquid water freezing into ice is a common example. As water cools, it loses heat. The particles slow down enough to form solid ice.

3. Vaporization

Vaporization happens when a liquid changes into a gas. This change needs the liquid to absorb energy. When particles gain enough energy, they can escape from the liquid and spread out as a gas.

There are two common ways this can happen:

• Evaporation: happens at the surface of a liquid, like a puddle drying up.
• Boiling: happens throughout the liquid when it gets hot enough, like water boiling in a pot.

Both evaporation and boiling are forms of vaporization.

4. Condensation

Condensation happens when a gas changes into a liquid. This happens when the gas releases energy. As particles lose energy, they 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 down, loses energy, and turns into liquid water.

5. Sublimation

Sublimation happens when a solid changes directly into a gas without becoming a liquid first. This change happens when the solid absorbs energy.

Dry ice is a well-known example. Dry ice is solid carbon dioxide. Instead of melting into a liquid, it changes straight into a gas.

6. Deposition

Deposition happens when a gas changes directly into a solid without becoming a liquid first. This happens when the gas releases energy.

Frost forming on a cold window is an example of deposition. Water vapor in the air loses energy and turns directly into solid ice.

Energy and phase transitions

A very important part of phase transitions is whether energy is absorbed or released.

• Absorbs energy: melting, vaporization, sublimation
• Releases energy: freezing, condensation, deposition

You can organize them like this:

• Solid 92 Liquid: melting, absorbs energy
• Liquid 92 Solid: freezing, releases energy
• Liquid 92 Gas: vaporization, absorbs energy
• Gas 92 Liquid: condensation, releases energy
• Solid 92 Gas: sublimation, absorbs energy
• Gas 92 Solid: deposition, releases energy

A simple pattern can help you remember this:

Moving toward gas means matter usually absorbs energy. Moving toward solid means matter usually releases energy.

Why temperature matters

Temperature tells us how much energy particles have. When temperature goes up, particles usually move faster. When temperature goes down, particles usually move slower.

If enough energy is added or removed, a phase transition can happen. For water, melting happens at about \(0^\circ\text{C}\), and boiling happens at about \(100^\circ\text{C}\) under normal conditions.

You do not need to memorize many numbers for every substance. The big idea is that each material changes state at certain temperatures.

Worked Example 1: Ice on a warm table

Question: A cube of ice is left on a warm table. After some time, it turns into liquid water. What phase transition happened, and did the ice absorb or release energy?

Step 1: Identify the starting state. Ice is a solid.

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

Step 3: Match the change. Solid 92 liquid is melting.

Step 4: Decide what happened to energy. Melting means the matter absorbed energy.

Answer: The phase transition was melting, and the ice absorbed energy.

Worked Example 2: Water droplets on a cold can

Question: On a hot day, drops of water form on the outside of a cold soda can. What phase transition happened?

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

Step 2: Water vapor is a gas.

Step 3: The droplets are liquid water.

Step 4: Gas 92 liquid is condensation.

Step 5: Condensation happens when matter releases energy.

Answer: The phase transition was condensation, and the water vapor released energy.

Worked Example 3: Dry ice in a science demo

Question: A teacher places dry ice on a tray. It seems to disappear and turn into a gas without making a puddle. What phase transition is this?

Step 1: Dry ice starts as a solid.

Step 2: It changes directly into a gas.

Step 3: Solid 92 gas is sublimation.

Step 4: Sublimation means the matter absorbed energy.

Answer: This is sublimation, and the dry ice absorbed energy.

Worked Example 4: Frost on the grass

Question: On a very cold morning, frost appears on the grass. Water vapor in the air turned directly into ice. What phase transition happened?

Step 1: Water vapor is a gas.

Step 2: Frost is a solid.

Step 3: Gas 92 solid is deposition.

Step 4: Deposition means the matter released energy.

Answer: The phase transition was deposition, and the water vapor released energy.

Common mistakes to avoid

• Melting is not the same as dissolving. Melting is a change of state, like ice turning to water. Dissolving is when one substance mixes into another, like sugar in water.
• Condensation does not mean water is leaking. Often, the liquid formed on a cold surface comes from water vapor in the air.
• Boiling and evaporation are both vaporization. They both make a liquid become a gas.
• Sublimation and deposition skip the liquid state. That is what makes them different from the other phase changes.

Quick review chart

• Melting: solid 92 liquid, absorbs energy
• Freezing: liquid 92 solid, releases energy
• Vaporization: liquid 92 gas, absorbs energy
• Condensation: gas 92 liquid, releases energy
• Sublimation: solid 92 gas, absorbs energy
• Deposition: gas 92 solid, releases energy

Brief summary

Phase transitions are physical changes in matter. They happen when matter moves between solid, liquid, and gas because energy is added or removed.

If matter absorbs energy, it can melt, vaporize, or sublimate. If matter releases energy, it can freeze, condense, or go through deposition. Learning the starting state, ending state, and energy change will help you identify any phase transition.

Thermal Expansion is what happens when matter gets bigger because it is heated.

When a material gains thermal energy from heat, its particles move faster. As the particles move faster, they usually spread a little farther apart. This causes the material to take up more space, or expand.

Most solids, liquids, and gases expand when they are heated and contract when they are cooled. Contract means to become smaller.

This idea helps explain many things we see in everyday life, like why bridges have small gaps, why lids loosen under warm water, and why hot air balloons rise.

Why does thermal expansion happen?

All matter is made of tiny particles. These particles are always moving, even if we cannot see them.

When heat is added, the particles gain energy and move more. In most materials, the extra movement makes the particles push a little farther apart from each other.

When the particles are farther apart, the object becomes slightly larger. This is thermal expansion.

When heat is removed, particles slow down and usually move closer together. Then the material contracts.

Thermal expansion in different states of matter

1. Solids

In a solid, particles are packed closely together and mostly vibrate in place. When a solid is heated, its particles vibrate faster and need a little more space. So the solid expands a small amount.

Solids usually expand less than liquids and gases, but even a tiny change can matter.

• Railroad tracks need small gaps so they do not bend on hot days.
• Bridges use expansion joints so they can safely expand and contract.
• A metal jar lid can loosen when warm water heats it and makes it expand.

2. Liquids

In a liquid, particles can move around each other. When a liquid is heated, its particles move faster and spread out more. So the liquid expands.

This is why the level of liquid in a thermometer rises when it gets warmer.

3. Gases

Gas particles are already far apart and move freely. When a gas is heated, the particles move much faster and spread out even more. Because of this, gases usually expand more than solids or liquids.

This is why hot air balloons work. Heating the air inside the balloon makes the air expand and become less dense, so the balloon rises.

Important idea: Heating does not create more matter

When something expands, it does not gain more matter. The same amount of matter is still there.

What changes is the space between the particles. The particles spread out more, so the object takes up more space.

Thermal expansion and cooling

Heating usually causes expansion. Cooling usually causes contraction.

For example:

• A metal spoon may become slightly longer when heated.
• The same spoon becomes slightly smaller again when it cools.

The special case of water

Water is unusual. Most substances contract as they cool. Water does this too, but only down to a certain temperature.

As liquid water cools from warm temperatures down to about \(4^\circ\text{C}\), it contracts like most substances.

But when water cools from about \(4^\circ\text{C}\) to \(0^\circ\text{C}\), it begins to expand instead of continuing to contract.

When water freezes into ice, it expands even more. That is why ice takes up more space than the same amount of liquid water.

This unusual behavior is called the anomalous behavior of water. Anomalous means unusual or different from what is expected.

Why is this important?

• Ice floats on liquid water because ice is less dense.
• Lakes freeze on the top first, which helps living things survive underwater.
• Water in cracks can freeze, expand, and break rocks or sidewalks over time.

Everyday examples of thermal expansion

1. Bridges and sidewalks: Engineers leave spaces so materials can expand on hot days without cracking.
2. Power lines: Wires sag more in summer because heat makes them expand.
3. Jar lids: A metal lid expands when warmed, which can make it easier to open.
4. Thermometers: The liquid inside expands when heated and rises up the tube.
5. Hot air balloons: Heated air expands, helping the balloon lift.

Worked Example 1: A heated metal rod

A metal rod is left in the Sun. What happens to its particles, and what happens to the rod?

Step 1: The Sun heats the rod, so it gains thermal energy.

Step 2: The particles in the rod vibrate faster.

Step 3: The particles move slightly farther apart.

Answer: The rod expands a little and becomes slightly longer.

Worked Example 2: Liquid in a thermometer

A thermometer is moved from a cool room to a warm room. Why does the liquid level rise?

Step 1: The liquid in the thermometer gains heat.

Step 2: Its particles move faster.

Step 3: The particles spread out more, so the liquid expands.

Answer: Because the liquid expands, it rises higher in the narrow tube.

Worked Example 3: Railroad tracks on a hot day

Why do railroad tracks have small gaps between sections?

Step 1: Metal tracks heat up in hot weather.

Step 2: The particles in the metal move faster and spread slightly apart.

Step 3: The metal expands.

Answer: The gaps give the metal room to expand so the tracks do not bend or buckle.

Worked Example 4: Water in a freezer

A glass bottle is filled completely with water and placed in a freezer. What might happen?

Step 1: The water cools.

Step 2: As it freezes, water expands instead of shrinking.

Step 3: The expanding ice pushes against the bottle.

Answer: The bottle may crack or break because frozen water takes up more space.

Common mistakes to avoid

• Mistake: Thinking heated objects expand because they gain more matter.
  Correct idea: They expand because their particles spread out.
• Mistake: Thinking all substances act exactly like water.
  Correct idea: Water is unusual near freezing.
• Mistake: Thinking only solids expand.
  Correct idea: Solids, liquids, and gases can all expand when heated.
• Mistake: Thinking expansion is always easy to see.
  Correct idea: Sometimes the change is very small, but still important.

Key ideas to remember

• Thermal expansion means matter gets bigger when heated.
• Heating gives particles more energy, so they move faster.
• In most materials, faster-moving particles spread farther apart.
• Cooling usually causes contraction.
• Solids, liquids, and gases can all expand.
• Water is special because it expands as it freezes.

Brief Summary

Thermal expansion happens when heat gives particles more energy, causing them to move faster and spread farther apart. This makes most materials expand when heated and contract when cooled.

Solids, liquids, and gases all show thermal expansion, but gases usually expand the most. Water is unusual because it expands when it freezes, which is why ice floats and frozen water can crack containers.

Intensive and Extensive Properties

Everything around us is made of matter. Matter is anything that has mass and takes up space. Scientists describe matter by looking at its properties, or characteristics.

Some properties change when you have more or less of a substance. Other properties stay the same no matter how much of the substance you have. These two groups are called extensive properties and intensive properties.

Learning the difference helps us describe, compare, and identify different kinds of matter.

1. What are extensive properties?

Extensive properties depend on the amount of matter you have.

If you use more of a substance, the value of an extensive property usually gets bigger. If you use less, it gets smaller.

• Mass — how much matter is in an object
• Volume — how much space an object takes up
• Length — how long something is
• Weight — how strongly gravity pulls on an object

For example, a small cup of water has less mass and less volume than a large bucket of water. That is why mass and volume are extensive properties.

2. What are intensive properties?

Intensive properties do not depend on the amount of matter you have.

If you split a substance into smaller pieces, the intensive properties stay the same.

• Density — how much mass is packed into a certain volume
• Boiling point — the temperature at which a liquid turns to gas
• Melting point — the temperature at which a solid turns to liquid
• Color — what the substance looks like
• Odor — how it smells

For example, a drop of pure water and a whole bottle of pure water both boil at the same temperature 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 have more of the substance, will this property change?”

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

Here is another way to think about it:

• Extensive = depends on amount
• Intensive = stays the same

4. Why density is intensive

Density can seem tricky at first. Density compares mass and volume.

The formula for density is:

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

If both the mass and volume increase in the same way, the density stays the same for the same substance.

For example, if one block of wood is twice as large as another block of the same wood, it will usually have about twice the mass and twice the volume. But the density stays the same.

That is why density is an intensive property, even though mass and volume are extensive properties.

5. Worked Examples

Example 1: Classifying mass

A rock has a mass of 200 grams. A bigger rock of the same type has a mass of 400 grams.

Question: Is mass intensive or extensive?

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

Step 2: The bigger rock has more matter, and its mass is greater.

Answer: Mass is an extensive property because it depends on how much matter there is.

Example 2: Classifying boiling point

You have a small pot of pure water and a large pot of pure water.

Question: Is boiling point intensive or extensive?

Step 1: Compare the property for different amounts of the same substance.

Step 2: The amount of water is different, but the boiling point stays the same under the same conditions.

Answer: Boiling point is an intensive property.

Example 3: Using density

A sample of a liquid has a mass of 10 grams and a volume of 5 milliliters.

We can find the density:

$$\text{density} = \frac{10}{5} = 2\,\text{g/mL}$$

Now imagine a larger sample of the same liquid has a mass of 20 grams and a volume of 10 milliliters.

Its density is:

$$\text{density} = \frac{20}{10} = 2\,\text{g/mL}$$

Question: What does this show?

Answer: Even though the mass and volume changed, the density stayed the same. So density is an intensive property.

Example 4: Sorting properties

A student is given these properties of a metal sample:

• mass = 50 g
• volume = 20 mL
• color = silver
• melting point = 660°C

Question: Which are intensive and which are extensive?

Step 1: Check which properties depend on the amount.

• Mass changes if you have more metal, so it is extensive.
• Volume changes if you have more metal, so it is extensive.

Step 2: Check which properties stay the same.

• Color stays the same, so it is intensive.
• Melting point stays the same, so it is intensive.

Answer:

• Extensive: mass, volume
• Intensive: color, melting point

6. Comparing the two types

Here is a quick comparison:

• Extensive properties depend on the amount of substance.
• Intensive properties do not depend on the amount of substance.

Examples of extensive properties:

• mass
• volume
• length
• weight

Examples of intensive properties:

• density
• boiling point
• melting point
• color
• odor

7. Common mistake to avoid

A common mistake is thinking that any property with a number must be extensive. That is not true.

For example, boiling point and density both use numbers, but they are still intensive because they stay the same for the same substance.

Another common mistake is mixing up mass and density. Mass depends on how much matter there is, but density tells how tightly packed the matter is. So mass is extensive, while density is intensive.

8. Why this matters

Scientists use intensive properties to help identify substances. For example, if an unknown liquid has the same density and boiling point as a known liquid, that gives clues about what it might be.

Scientists use extensive properties to describe how much of a substance is present. For example, mass and volume can tell how much water is in a container.

Summary

Properties of matter can be sorted into two groups.

• Extensive properties depend on the amount of matter. Examples: mass and volume.
• Intensive properties do not depend on the amount of matter. Examples: density and boiling point.

To classify a property, ask: Does it change when the amount changes?

• If yes, it is extensive.
• If no, it is intensive.

Once you remember that idea, it becomes much easier to sort the properties of matter correctly.

Pure Substances are kinds of matter that are made of only one type of particle and have a fixed, uniform composition. This means the substance is the same all the way through, and every sample of it is made of the same material.

Learning about pure substances helps us sort and describe matter. It also helps us tell the difference between elements, compounds, and mixtures.

In this lesson, you will learn:

• What a pure substance is
• The two types of pure substances: elements and compounds
• How pure substances are different from mixtures
• How to recognize examples of each

1. What is a pure substance?

A pure substance is matter that has a constant composition. That means it always contains the same ingredients in the same amount.

For example, a sample of pure water is always made of water particles. A sample of pure oxygen is always made of oxygen particles. These substances do not have other materials mixed in with them.

Pure substances have the same properties throughout the sample. If you take one part from the top and one part from the bottom, they are still the same substance.

Important idea: A pure substance is not just something that looks clean. In science, “pure” means it is made of only one substance.

2. The two types of pure substances

There are two main kinds of pure substances:

• Elements
• Compounds

A. 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
• Copper

If you had a pure sample of gold, every part of it would be gold. If you had a pure sample of oxygen gas, every part would be oxygen.

B. Compounds

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

Examples of compounds include:

• Water
• Carbon dioxide
• Table salt

In a compound, the elements are joined together in the same ratio every time. For example, water is made from hydrogen and oxygen in a fixed ratio:

$$\text{Water} = \text{H}_2\text{O}$$

This means each water particle has 2 hydrogen atoms and 1 oxygen atom. Every pure sample of water has that same composition.

Table salt is another compound:

$$\text{NaCl}$$

This means sodium and chlorine are joined together in a fixed way.

3. Fixed composition means the parts do not change

One of the most important features of a pure substance is that its composition is fixed. That means the substance is always made of the same particles.

For example:

• Pure oxygen is only oxygen
• Pure gold is only gold
• Pure water is always \(\text{H}_2\text{O}\)

You cannot have a pure substance that sometimes has one makeup and sometimes has another. If the makeup changes from place to place, then it is not a pure substance.

4. Pure substances and mixtures

A mixture is made when two or more substances are combined without being chemically joined into one new substance.

In a mixture:

• The parts can be in different amounts
• The composition can vary
• The substances keep many of their own properties

Examples of mixtures:

• Salt water
• Trail mix
• Air
• Soil

Salt water is not a pure substance because the amount of salt can change. One cup may have a little salt, and another cup may have a lot. That means the composition is not fixed.

Air is also a mixture because it contains several gases, such as nitrogen and oxygen, mixed together.

5. How to tell the difference between an element, compound, and mixture

Use these questions:

1. Is it made of only one kind of material all the way through?
2. If yes, is it made of one kind of atom or more than one kind of atom?
3. If it has more than one substance physically combined, is it a mixture?

Here is a simple way to think about it:

• Element: one kind of atom only
• Compound: two or more elements chemically joined in a fixed ratio
• Mixture: two or more substances combined, but not chemically joined

6. Everyday examples

Pure substance examples:

• A piece of pure copper wire
• A balloon filled with pure helium
• Pure water in a science lab
• A crystal of pure table salt

Not pure substance examples:

• Lemonade
• Milk
• Salad
• Muddy water

Lemonade has water, sugar, and lemon juice. Milk contains water, fats, proteins, and other substances. Salad has many different parts. Muddy water has water mixed with dirt. These are all mixtures.

7. Worked Examples

Example 1: Is oxygen a pure substance?

Step 1: Ask what oxygen is made of.

It is made of only oxygen particles.

Step 2: Decide whether the composition stays the same.

Yes. A pure sample of oxygen is the same all the way through.

Answer: Yes, oxygen is a pure substance. It is an element.

Example 2: Is water a pure substance?

Step 1: Ask whether water has a fixed composition.

Yes. Pure water is always \(\text{H}_2\text{O}\).

Step 2: Decide whether it is an element or compound.

It has hydrogen and oxygen joined together, so it is not a single element.

Answer: Yes, water is a pure substance. It is a compound.

Example 3: Is salt water a pure substance?

Step 1: Identify what is in salt water.

It contains water and salt.

Step 2: Ask whether the amount of salt is always the same.

No. Some salt water has more salt, and some has less.

Step 3: Decide if the composition is fixed.

No. The composition can change.

Answer: No, salt water is not a pure substance. It is a mixture.

Example 4: Classify gold, carbon dioxide, and trail mix

Gold: made of one kind of atom only.

Classification: pure substance, element.

Carbon dioxide: made of carbon and oxygen chemically joined together.

Classification: pure substance, compound.

Trail mix: made of different foods combined together.

Classification: mixture, not a pure substance.

8. Common mistakes to avoid

• Mistake: Thinking “pure” means safe or healthy.
  In science, pure means made of only one substance.
• Mistake: Thinking all clear liquids are pure substances.
  Many clear liquids, like vinegar or soda, are mixtures.
• Mistake: Thinking compounds are not pure because they contain more than one element.
  A compound is a pure substance because the elements are chemically joined in a fixed ratio.

9. Quick check for understanding

• Is an element a pure substance? Yes.
• Is a compound a pure substance? Yes.
• Is a mixture a pure substance? No.
• Does a pure substance have a fixed composition? Yes.

Summary

A pure substance is matter made of only one kind of material and has a fixed, uniform composition. There are two types of pure substances: elements, which are made of one kind of atom, and compounds, which are made of two or more elements chemically joined in a fixed ratio.

Mixtures are different because their composition can change. To classify matter, ask whether it is the same all the way through and whether its makeup stays constant.

Mixtures are made when two or more kinds of matter are combined, but not chemically changed into a new substance.

This means each part of the mixture keeps its own properties. For example, if you mix sand and water, the sand is still sand and the water is still water.

Learning about mixtures helps us describe matter, sort materials into groups, and understand many things we use every day, like air, salad, milk, and muddy water.

In this lesson, you will learn what a mixture is, the two main kinds of mixtures, and how to tell them apart.

What is a mixture?

A mixture is a physical blend of substances. A physical blend means the substances are together, but they are not joined into a new substance.

Mixtures can often be separated by physical methods, such as:

• sorting by hand
• filtering
• letting parts settle
• evaporating
• using a magnet, if one part is magnetic

For example, a mixture of iron filings and sand can be separated with a magnet. Salt water can be separated by evaporating the water and leaving the salt behind.

Important idea: In a mixture, the amount of each substance can change. You can have a little sugar in water or a lot of sugar in water, and it is still a mixture.

Two main types of mixtures

Mixtures are often grouped as homogeneous or heterogeneous.

1. Homogeneous mixtures

A homogeneous mixture looks the same all the way through. The parts are spread out evenly, so you cannot easily see the different substances.

A homogeneous mixture is also called a solution.

In a solution:

• one substance dissolves in another
• the mixture looks uniform
• the particles do not settle out
• you usually cannot separate the parts by simple filtering

Examples of homogeneous mixtures:

• salt water
• sugar water
• air
• vinegar
• some metal mixtures, like brass

Salt water looks like one substance because the salt spreads evenly through the water. Even though you cannot see the salt, it is still there.

2. Heterogeneous mixtures

A heterogeneous mixture does not look the same all the way through. You can often see different parts or pieces.

In a heterogeneous mixture:

• the substances are not spread out evenly
• different parts may be visible
• some parts may settle over time
• the mixture may often be separated more easily

Two common kinds of heterogeneous mixtures are suspensions and colloids.

Suspensions

A suspension is a heterogeneous mixture in which small particles are mixed through a liquid or gas, but the particles are large enough to settle out over time.

Examples of suspensions:

• muddy water
• sand in water
• orange juice with pulp

If muddy water sits still, the dirt sinks to the bottom. That tells us it is a suspension.

Colloids

A colloid is a heterogeneous mixture with tiny particles spread throughout another substance. These particles are small enough that they do not settle out easily.

Colloids may look smooth or even uniform at first, but they are still not true solutions.

Examples of colloids:

• milk
• fog
• whipped cream
• gelatin

Milk may look the same throughout, but it contains tiny particles spread through liquid. Those particles do not settle quickly, so milk is a colloid.

How to tell the types of mixtures apart

Here are some questions you can ask:

1. Does it look the same all the way through?
   If yes, it may be homogeneous.
2. Can you see different parts?
   If yes, it is likely heterogeneous.
3. Do the particles settle out over time?
   If yes, it is probably a suspension.
4. Does it seem smooth, but it is not a true solution?
   If yes, it may be a colloid.

Mixtures compared with pure substances

A pure substance is made of only one kind of matter. It has the same composition all the way through.

A mixture contains more than one substance physically combined.

For example:

• Pure substance: pure water
• Mixture: salt water

Pure water is only water. Salt water contains water and salt together.

Why mixtures matter in daily life

Many materials around us are mixtures. Understanding mixtures helps us explain what we eat, drink, breathe, and use.

• Air is a homogeneous mixture of gases.
• Trail mix is a heterogeneous mixture because you can see each part.
• Salad dressing can be a suspension if particles settle.
• Milk is a colloid.

Knowing the type of mixture also helps us know how to separate it or use it.

Worked Example 1: Salt water

Question: Is salt water homogeneous or heterogeneous?

Step 1: Ask if it looks the same throughout.

Yes. When salt dissolves well, you cannot see separate pieces of salt.

Step 2: Ask if the salt settles out quickly.

No. The salt stays spread out in the water.

Answer: Salt water is a homogeneous mixture, also called a solution.

Worked Example 2: Sand and water

Question: Is sand and water homogeneous or heterogeneous?

Step 1: Can you see different parts?

Yes. You can see the sand and the water.

Step 2: Does the sand settle?

Yes. If the mixture sits still, the sand sinks to the bottom.

Answer: Sand and water is a heterogeneous mixture. More specifically, it is a suspension.

Worked Example 3: Milk

Question: Milk looks smooth. Is it a solution?

Step 1: Does milk appear uniform?

Yes, it often does.

Step 2: Is it a true solution with everything dissolved?

No. Milk has tiny particles spread through it.

Step 3: Do the particles settle quickly?

No, not usually.

Answer: Milk is a heterogeneous mixture called a colloid.

Worked Example 4: Classifying everyday mixtures

Question: Classify each mixture: air, muddy water, and trail mix.

Air: It looks the same throughout and is made of gases mixed evenly. It is a homogeneous mixture.

Muddy water: It has particles that can settle. It is a heterogeneous mixture, specifically a suspension.

Trail mix: You can clearly see different parts, like nuts and raisins. It is a heterogeneous mixture.

Quick comparison chart

• Homogeneous mixture (solution): looks the same throughout; particles do not settle; example: salt water
• Heterogeneous mixture: does not look the same throughout; parts may be visible; example: salad
• Suspension: a heterogeneous mixture with particles that settle; example: muddy water
• Colloid: a heterogeneous mixture with tiny particles that do not settle easily; example: milk

Tips for answering questions about mixtures

• Look closely: can you see different parts?
• Think about time: will some parts settle?
• Ask if one substance dissolves evenly into another.
• Remember that not everything that looks smooth is a solution.

Summary

A mixture is a physical blend of two or more substances. The substances keep their own properties and can often be separated by physical means.

Homogeneous mixtures look the same all the way through and are called solutions. Examples include salt water and air.

Heterogeneous mixtures do not look the same all the way through. Suspensions have particles that settle, like muddy water. Colloids have tiny particles that do not settle easily, like milk.

If you remember to ask "Does it look the same throughout?" and "Do particles settle out?", you can classify most mixtures correctly.

Solubility and Saturation are important ideas in science because they help us understand how substances mix together. When you stir sugar into water, some of the sugar seems to disappear. It does not vanish. Instead, it dissolves in the water.

In this lesson, you will learn what solute, solvent, solution, solubility, and saturation mean. You will also learn how temperature and pressure can change how much of a substance dissolves.

Let’s begin with the basic parts of a solution.

• Solute: the substance that gets dissolved.
• Solvent: the substance that does the dissolving.
• Solution: the mixture made when the solute dissolves in the solvent.

For example, if you mix salt with water, the salt is the solute, the water is the solvent, and the salt water is the solution.

Solubility means how much of a solute can dissolve in a certain amount of solvent. Different substances have different solubilities. For example, sugar dissolves well in water, but sand does not.

If a solute dissolves easily, we say it is soluble. If it does not dissolve well, we say it is insoluble.

Scientists often compare solubility by asking questions like: “How many grams of sugar can dissolve in 100 grams of water?” You do not need to memorize exact numbers, but it is helpful to know that solubility is about the maximum amount that can dissolve.

Saturation happens when the solvent has dissolved as much solute as it can at a certain temperature. After that point, extra solute will not dissolve and will stay at the bottom or float around in the liquid.

There are three common ways to describe a solution:

• Unsaturated: more solute can still dissolve.
• Saturated: the solvent has dissolved the maximum amount possible.
• Supersaturated: more solute is dissolved than normally possible, usually because the solution was heated and then cooled carefully.

For 6th Grade science, the most important ideas are unsaturated and saturated.

Imagine adding spoonfuls of sugar to a cup of water.

1. At first, the sugar dissolves quickly.
2. After more and more sugar is added, it dissolves more slowly.
3. Eventually, some sugar stays at the bottom.

At that point, the solution is saturated. The water cannot dissolve any more sugar unless something changes, such as the temperature.

Temperature often affects solubility. Temperature tells us how hot or cold something is. In many cases, solid solutes dissolve better in warmer liquids than in colder liquids.

For example, sugar usually dissolves faster and in greater amounts in hot tea than in iced tea. That is because warmer water has more energy, which helps the sugar particles spread out and mix into the water.

This means that if you heat a liquid, it can often dissolve more of a solid solute. If you cool the liquid, it may be able to hold less dissolved solute.

That is why crystals can form when a hot solution cools down. The solvent can no longer hold as much dissolved solute, so some of the solute comes out of the solution.

For many gases, temperature has the opposite effect. Gases usually dissolve better in colder liquids than in warmer ones.

A common example is a cold soda. Soda contains carbon dioxide gas dissolved under pressure. When the soda gets warm, the gas escapes more easily, so warm soda often seems flatter than cold soda.

Pressure has a strong effect on the solubility of gases. Pressure is the push of particles against a surface. When pressure above a liquid increases, more gas can usually dissolve in that liquid.

This is why soda is bottled under pressure. The high pressure helps keep carbon dioxide gas dissolved in the drink. When you open the bottle, the pressure suddenly drops, and bubbles of gas rush out.

Pressure does not affect solids dissolving in liquids very much in everyday situations. In 6th Grade science, remember this simple rule:

• Temperature strongly affects solids in liquids.
• Pressure strongly affects gases in liquids.

Let’s look more closely at how to tell whether a solution is unsaturated or saturated.

If you add a little solute and it disappears completely after stirring, the solution is probably unsaturated. If you keep adding solute and some remains undissolved, the solution is saturated.

It is important to remember that dissolving is a physical change, not a chemical change. The solute is still there, even though it looks like it disappeared. For example, if water evaporates from salt water, the salt will remain behind.

Now let’s work through some examples.

Worked Example 1: Identifying solute, solvent, and solution

A student mixes 1 spoon of salt into a glass of water and stirs until the salt disappears.

• What is the solute? Salt
• What is the solvent? Water
• What is the solution? Salt water

Answer: The salt is the solute because it dissolves. The water is the solvent because it does the dissolving. Together they make a salt-water solution.

Worked Example 2: Unsaturated or saturated?

Maria adds sugar to warm water. The first 3 spoonfuls dissolve. On the 4th spoonful, some sugar remains at the bottom even after stirring.

Step 1: Ask whether all the sugar dissolved. No.

Step 2: If extra solute stays undissolved, the solution has reached its limit.

Answer: The solution is saturated.

Worked Example 3: Effect of temperature on a solid

Two cups contain the same amount of water. One cup is hot, and one cup is cold. You add sugar to both cups.

Question: In which cup will more sugar probably dissolve?

Reasoning: Most solid solutes dissolve better in warmer liquids.

Answer: More sugar will probably dissolve in the hot water.

You can think of it like this:

Hot water can hold more dissolved sugar than cold water.

Worked Example 4: Effect of pressure on a gas

A sealed bottle of soda is opened. Right away, bubbles form and rise to the top.

Question: Why do the bubbles appear after the bottle is opened?

Step 1: In the sealed bottle, carbon dioxide gas is dissolved under high pressure.

Step 2: Opening the bottle lowers the pressure.

Step 3: Lower pressure means the liquid cannot hold as much gas.

Answer: The gas comes out of the solution, forming bubbles.

Sometimes students confuse melting with dissolving. These are different processes.

• Melting: a solid changes to a liquid because of heat.
• Dissolving: a solute mixes evenly into a solvent to form a solution.

For example, ice melting into water is melting. Sugar disappearing into water is dissolving.

Here are some useful everyday examples of solubility and saturation:

• Making lemonade by dissolving sugar in water
• Adding chocolate powder to milk
• Seeing salt left behind after ocean water evaporates
• Watching bubbles escape from soda after opening it

Here are the main patterns to remember:

• Different solutes have different solubilities.
• A solution becomes saturated when no more solute can dissolve.
• Higher temperature usually helps more solid solute dissolve in a liquid.
• Higher pressure usually helps more gas dissolve in a liquid.
• When pressure drops, dissolved gas often escapes as bubbles.

If we wanted to write a simple comparison using math symbols, we could say:

For many solids in liquids:

$$\text{higher temperature} \rightarrow \text{higher solubility}$$

For gases in liquids:

$$\text{higher pressure} \rightarrow \text{higher solubility}$$

And when a solution is saturated, the amount dissolved is at its maximum for that temperature:

$$\text{dissolved solute} = \text{maximum amount the solvent can hold}$$

Quick Check for Understanding

• If salt keeps dissolving in water, is the solution saturated or unsaturated?
• Does hot water or cold water usually dissolve more sugar?
• Why does soda fizz when opened?
• What is the difference between a solute and a solvent?

Brief Summary

Solubility tells how much of a substance can dissolve in a solvent. The dissolved substance is the solute, and the substance doing the dissolving is the solvent. When no more solute can dissolve, the solution is saturated.

Temperature often affects solids dissolving in liquids, and warmer liquids usually dissolve more solid solute. Pressure mostly affects gases dissolved in liquids, and higher pressure usually keeps more gas dissolved. These ideas help explain everyday things like sweet tea, salt water, and fizzy soda.

Separation Techniques are ways to separate the parts of a mixture. A mixture is made when two or more substances are combined, but they do not change into a new substance. Because the substances keep their own properties, we can often separate them again.

Scientists use separation techniques every day. People also use them at home, in cooking, in cleaning water, and in medicine. To choose the best method, we look at the physical properties of the substances, such as particle size, boiling point, and how heavy the particles are.

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

• Filtration
• Distillation
• Chromatography
• Centrifugation

Each method works because the parts of a mixture are different in some way. If we know that difference, we can separate the mixture.

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, such as filter paper. The liquid passes through, but the solid particles are trapped.

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

For example, if you mix sand and water, the sand does not dissolve. When you pour the mixture through filter paper, the sand stays behind and the water passes through.

Filtration works because the sand particles are bigger than the tiny holes in the filter paper, while the water particles can pass through.

Filtration is useful for:

• Separating sand from water
• Making coffee with a coffee filter
• Cleaning some dirty water

2. Distillation

Distillation is used to separate substances in a liquid mixture by using their different boiling points.

The boiling point is the temperature at which a liquid changes into a gas. In distillation, the mixture is heated. The liquid with the lower boiling point evaporates first. The gas is then cooled and turns back into a liquid. This process is called condensation.

So distillation has two main steps:

1. Heat the liquid so one part evaporates.
2. Cool the gas so it condenses back into a liquid.

A common example is separating pure water from salty water. When salty water is heated, the water evaporates, but the salt does not. The water vapor is collected and cooled, producing liquid water without the salt.

Distillation works because salt and water have very different boiling points. Water can evaporate while the salt stays behind.

Distillation is useful for:

• Making fresh water from salt water
• Purifying liquids
• Separating some liquids from each other

3. Chromatography

Chromatography is used to separate substances that are dissolved in a liquid. It is often used to separate different colors in ink.

In 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, but the spot starts above the liquid level. As the solvent moves up the paper, it carries the substances with it.

The different substances move at different speeds. Some dissolve better in the solvent, and some stick more strongly to the paper. Because of this, the parts of the mixture separate into different spots.

For example, black ink may actually be made of blue, red, and yellow dyes. Chromatography can separate these dyes so we can see the different colors.

Chromatography is useful for:

• Separating colors in ink
• Checking what substances are in a mixture
• Comparing samples

4. Centrifugation

Centrifugation is a method that spins a mixture very quickly to separate substances based on how heavy they are.

A machine called a centrifuge spins the mixture. Heavier particles move outward and settle faster, while lighter parts stay closer to the center or remain on top.

One example is blood. Blood is a mixture. When spun in a centrifuge, its parts separate into layers because they have different masses and densities.

Centrifugation is also used to separate tiny solid particles from liquids when filtration would be too slow or difficult.

Centrifugation is useful for:

• Separating parts of blood
• Separating cream from milk
• Separating fine particles from a liquid

How to Choose the Right Separation Technique

To choose a separation method, ask questions about the mixture:

• Is there an insoluble solid in a liquid? Use filtration.
• Do the substances have different boiling points? Use distillation.
• Are there dissolved colors or substances that move differently in a solvent? Use chromatography.
• Do parts of the mixture have different masses and need spinning? Use centrifugation.

These methods are based on physical properties, so they usually cause a physical change, not a chemical change. The substances are still the same substances after separation.

Worked Example 1: Sand and Water

Question: A student mixes sand and water. Which separation technique should be used?

Step 1: Identify the mixture. Sand is a solid, and water is a liquid.

Step 2: Decide if the solid dissolves. Sand does not dissolve in water.

Step 3: Choose the method. Since this is an insoluble solid in a liquid, use filtration.

Answer: The correct technique is filtration.

Worked Example 2: Salt Water

Question: A class wants to collect pure water from salt water. Which method should they use?

Step 1: Identify the mixture. Salt is dissolved in water.

Step 2: Think about filtration. Filtration will not remove dissolved salt because the salt particles are too small and move with the water.

Step 3: Think about boiling points. Water can evaporate and then condense, while salt stays behind.

Answer: The correct technique is distillation.

Worked Example 3: Colors in Marker Ink

Question: A student wants to find out whether a green marker contains more than one dye. Which method should be used?

Step 1: The student is testing a colored substance dissolved in ink.

Step 2: The goal is to separate the different dyes.

Step 3: The method used to separate dyes is chromatography.

Answer: The correct technique is chromatography.

Worked Example 4: Separating Blood into Parts

Question: A scientist needs to separate blood into layers. Which technique should be used?

Step 1: Blood is a liquid mixture with parts of different masses.

Step 2: The method must separate parts by spinning.

Step 3: The correct method is centrifugation.

Answer: The correct technique is centrifugation.

Comparing the Four Methods

• Filtration: separates an insoluble solid from a liquid using a filter.
• Distillation: separates substances by different boiling points using heating and cooling.
• Chromatography: separates dissolved substances because they move at different speeds.
• Centrifugation: separates substances by spinning them based on mass and density.

Important Ideas to Remember

• A mixture can often be separated because its parts keep their own properties.
• The best separation method depends on the physical properties of the substances.
• Filtration does not remove dissolved substances.
• Distillation can collect a liquid after it evaporates and condenses.
• Chromatography is very useful for separating colors.
• Centrifugation is helpful when tiny particles need to be separated quickly.

Brief Summary

Separation techniques help us separate the parts of mixtures. Filtration separates insoluble solids from liquids, distillation separates substances by boiling point, chromatography separates dissolved substances like dyes, and centrifugation separates materials by spinning them. By looking at the properties of a mixture, we can choose the best method to separate it.

Physical and Chemical Changes

Everything around us is made of matter. Matter can change in many ways. Sometimes the change only affects how something looks, feels, or is shaped. Other times, the change creates a new substance. Learning the difference between these two kinds of changes helps us understand what is happening in the world around us.

In this lesson, you will learn about physical changes and chemical changes, how to tell them apart, and what clues to look for.

1. What is a physical change?

A physical change is a change in the form of matter that does not create a new substance. The material is still the same substance as before, even if it looks different.

Physical changes can affect:

• size
• shape
• state of matter
• texture
• appearance

For example, if you tear paper into smaller pieces, it is still paper. If ice melts into water, it is still the same substance, just in a different state.

Common examples of physical changes:

• cutting a sheet of paper
• breaking a pencil
• melting ice
• freezing water
• boiling water
• dissolving sugar in water
• crushing a can

2. What is a chemical change?

A chemical change happens when matter changes into a new substance with new properties. After a chemical change, the original substance is no longer exactly the same.

Chemical changes often happen because tiny particles in matter rearrange to form something new. You do not need to see the particles to know a chemical change happened. Instead, you can look for clues.

Common signs of a chemical change:

• a color change
• a gas is produced, such as bubbles not caused by boiling
• a solid forms from liquids being mixed
• heat or light is produced
• a new smell appears

Common examples of chemical changes:

• rust forming on iron
• wood burning
• an egg cooking
• baking a cake
• milk souring
• fruit rotting

3. A very important question: Is a new substance formed?

The best way to decide whether a change is physical or chemical is to ask:

Did this change create a new substance?

If the answer is no, it is usually a physical change.

If the answer is yes, it is a chemical change.

4. Changes of state are physical changes

Matter can change from one state to another:

• solid to liquid: melting
• liquid to solid: freezing
• liquid to gas: evaporation or boiling
• gas to liquid: condensation

These are all physical changes because the substance stays the same. Water is a good example:

• ice = solid water
• liquid water = water
• water vapor = gas water

Even though the state changes, the substance is still water.

5. Physical change or chemical change? Look carefully.

Sometimes changes can be confusing. A color change or bubbles can be clues, but you must think about what caused them.

For example, when water boils, you see bubbles. But those bubbles are just water changing from liquid to gas. No new substance is formed, so boiling water is a physical change.

But if you mix two substances and bubbles form because a gas is made, that can be a chemical change.

6. Worked Examples

Example 1: Ice melting on a table

Question: Is melting ice a physical change or a chemical change?

Step 1: What changed? The ice changed from a solid to a liquid.

Step 2: Did a new substance form? No. It is still water.

Answer: This is a physical change.

Example 2: A bicycle left in the rain begins to rust

Question: Is rusting a physical change or a chemical change?

Step 1: What changed? The metal formed rust.

Step 2: Did a new substance form? Yes. Rust is different from the original metal.

Answer: This is a chemical change.

Example 3: Sugar dissolving in water

Question: Is dissolving sugar in water a physical change or a chemical change?

Step 1: The sugar seems to disappear, but it is still there in the water.

Step 2: Did a new substance form? No. The sugar is still sugar.

Answer: This is a physical change.

Example 4: Baking a cake

Question: Is baking a cake a physical change or a chemical change?

Step 1: Heat causes the ingredients to change.

Step 2: The batter becomes cake with different smell, texture, and taste.

Step 3: Did a new substance form? Yes, the ingredients changed into something new.

Answer: This is a chemical change.

7. Helpful comparison

• Physical change: changes appearance, size, shape, or state; no new substance forms
• Chemical change: creates a new substance with new properties

8. Quick clues chart

• If something is cut, bent, broken, melted, frozen, or dissolved, it is often a physical change.
• If something burns, rusts, cooks, rots, or forms gas from mixing, it is often a chemical change.

9. Be careful with reversibility

Some physical changes can be reversed easily. For example, water can freeze and then melt again. However, not all physical changes are easy to reverse. If you tear paper, it is still a physical change even though putting it back together is difficult.

Many chemical changes are hard or impossible to reverse in everyday life. You cannot easily turn a baked cake back into batter.

10. How to solve questions on your own

When you are asked whether something is a physical or chemical change, follow these steps:

1. Describe what changed.
2. Ask whether a new substance formed.
3. Look for clues like heat, light, gas, color change, or a new smell.
4. Decide if the change was only in form or if it changed the substance itself.

11. Practice thinking

Try these on your own:

• breaking a glass
• toasting bread
• freezing juice into a popsicle
• burning a marshmallow

Ask yourself each time: Did a new substance form?

12. Summary

A physical change changes the way matter looks or changes its state, but it does not make a new substance. A chemical change makes a new substance with new properties.

Remember the big idea: If no new substance forms, it is a physical change. If a new substance forms, it is a chemical change. This simple question can help you solve many science problems about matter.

Law of Conservation of Mass

The Law of Conservation of Mass says that matter is not created or destroyed. It can change form, change state, or be rearranged in a chemical reaction, but the total amount of matter stays the same.

In simple words, if you start with a certain amount of stuff, you end with the same total amount of stuff, even if it looks different.

This idea is important in science because it helps us understand what happens when matter changes. Whether ice melts, water evaporates, or chemicals react, the total mass stays constant if all the matter is kept in the system.

What is mass?

Mass is the amount of matter in an object. It is often measured in grams or kilograms. Mass is different from size. A small object can still have a lot of mass if it is packed tightly with matter.

When scientists talk about conservation of mass, they compare the total mass before a change and after a change.

$$\text{mass before} = \text{mass after}$$

Mass in physical changes

A physical change happens when matter changes form or state, but it stays the same substance. For example, ice melting into liquid water is a physical change. The water is still water.

Other physical changes include:

• melting
• freezing
• evaporation
• condensation
• breaking or cutting

During these changes, the total mass stays the same. If 50 grams of ice melts, it becomes 50 grams of liquid water.

Mass in chemical changes

A chemical change happens when substances react and form new substances. Even though the materials may look very different after the reaction, the total mass still stays the same.

For example, if vinegar and baking soda react in a sealed container, the total mass before and after the reaction is equal. The materials change into new substances, including a gas, but no matter disappears.

Why does mass sometimes seem to change?

Sometimes it looks like mass is lost or gained. This usually happens because some matter, often a gas, leaves the container or comes in from the air.

Imagine burning a piece of paper. Afterward, there is only a little ash left, so it may seem like mass was destroyed. But some of the matter turned into gases and went into the air. If you could measure all the ash and gases, the total mass would equal the starting mass.

This is why scientists often use a closed system. A closed system is a container where matter cannot get in or out. In a closed system, it is easier to show that mass stays the same.

Key idea to remember

The law works for both:

• Physical changes — the substance stays the same, but its form or state changes.
• Chemical changes — new substances form, but the total mass stays constant.

Worked Example 1: Melting ice

A student measures 30 grams of ice in a cup. The ice melts completely into liquid water. What is the mass of the water?

Step 1: Identify the type of change. Melting is a physical change.

Step 2: Use the law of conservation of mass.

$$\text{mass before} = \text{mass after}$$

$$30\text{ g} = \text{mass of water after melting}$$

Answer: The water has a mass of 30 grams.

Worked Example 2: Dissolving sugar in water

A cup holds 100 grams of water. A student adds 20 grams of sugar and stirs until the sugar dissolves. What is the total mass of the sugar-water mixture?

Step 1: Add the masses of the starting materials.

$$100\text{ g} + 20\text{ g} = 120\text{ g}$$

Step 2: Remember that dissolving is a physical change. The sugar seems to disappear, but it is still there in the mixture.

Answer: The total mass of the mixture is 120 grams.

Worked Example 3: A chemical reaction in a sealed bag

A sealed plastic bag contains 10 grams of one substance and 15 grams of another substance. They react and form new substances. What is the total mass after the reaction?

Step 1: Find the total mass before the reaction.

$$10\text{ g} + 15\text{ g} = 25\text{ g}$$

Step 2: Because the bag is sealed, no matter can leave or enter.

Answer: The total mass after the reaction is 25 grams.

Worked Example 4: When mass seems to disappear

A student mixes two liquids in an open cup. The mass before mixing is 80 grams. After the reaction, the cup and contents measure 74 grams. Did the law fail?

Step 1: Compare the masses.

Before: 80 g
After: 74 g

Step 2: Think about the system. The cup was open, so some matter may have left as a gas.

Step 3: The law of conservation of mass still applies. The missing 6 grams most likely became gas and escaped into the air.

$$80\text{ g} - 74\text{ g} = 6\text{ g}$$

Answer: No, the law did not fail. The system was open, so some matter left the cup.

How to use this law in science class

When you answer questions about conservation of mass, ask yourself:

1. What is the mass before the change?
2. What kind of change is it: physical or chemical?
3. Is the system closed or open?
4. Did any gas leave or enter?
5. Should the total mass stay the same?

Important facts to remember

• Matter is not created during a change.
• Matter is not destroyed during a change.
• Mass stays the same in both physical and chemical changes.
• If mass seems to change, some matter probably moved into or out of the system.
• Closed systems help show conservation of mass clearly.

Common mistakes

• Thinking matter disappears when it changes state, like water evaporating.
• Thinking dissolved materials no longer have mass.
• Thinking a chemical reaction destroys matter because new substances form.
• Forgetting that gases are matter and have mass.

Quick check

• If 45 grams of water freezes, the ice still has a mass of 45 grams.
• If 12 grams of powder are mixed with 8 grams of liquid in a sealed container, the total mass after mixing is 20 grams.
• If a reaction in an open cup seems to lose mass, a gas may have escaped.

Summary

The Law of Conservation of Mass says that matter is neither created nor destroyed. The total mass before a physical or chemical change is equal to the total mass after the change.

Even when matter changes state or forms new substances, the amount of matter stays the same. If the mass seems different, it is usually because the system was open and some matter, often a gas, moved in or out.