Properties and States of Matter

Defining Matter and Mass

Everything around us is made of matter. The air, your pencil, your water bottle, a rock, and even your body are all made of matter.

Matter is anything that has mass and takes up space.

Let’s learn what those words mean.

Mass means how much “stuff” is in an object. If something has more matter in it, it usually has more mass.

Space means the room an object takes up. If you put a book on a table, the book uses part of the table’s space. Water in a cup takes up space too. Even air takes up space, even though we usually cannot see it.

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

• have mass
• take up space

If it does both, it is matter.

Objects can be big or small, hard or soft, heavy or light. But if they have mass and take up space, they are all matter.

Examples of matter

• a chair
• a drop of water
• a balloon full of air
• a grain of sand
• a dog
• a cloud

Some things are not matter because they do not have mass and do not take up space in the same way objects do.

Examples of things that are not matter

• light
• sound
• heat
• a shadow

You can see light and hear sound, but they are not matter. They are forms of energy, not physical objects.

Matter can be easy or hard to see

Some matter is easy to see, like a desk or a basketball. Some matter is harder to notice, like air.

Air is matter because it has mass and takes up space. If you blow air into a balloon, the balloon gets bigger. That shows the air is taking up space inside the balloon.

Matter is everywhere

Everything physical in the universe is made of matter. Plants, animals, oceans, mountains, and the things in your classroom are all matter.

Even tiny pieces of matter still count as matter. A crumb, a raindrop, or a tiny bit of dust all have mass and take up space.

Mass is not the same as size

A bigger object often has more mass, but not always. A large beach ball may be bigger than a small rock, but the rock can have more mass.

That is because the rock has more matter packed into it.

How can we describe matter?

We can describe matter by its properties. For this lesson, the most important properties are:

• it has mass
• it takes up space

Later, you may also learn that matter can be solid, liquid, or gas. But all of those are still matter because they have mass and take up space.

Worked Example 1: Is a pencil matter?

Ask two questions:

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

A pencil has mass. You can hold it, and it is made of wood, paint, and graphite.

A pencil also takes up space in your desk or pencil box.

Answer: Yes, a pencil is matter.

Worked Example 2: Is air matter?

Air can be tricky because we usually cannot see it.

Ask the same two questions:

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

Yes. Air fills a balloon and makes it expand. That means air takes up space.

Air also has mass.

Answer: Yes, air is matter.

Worked Example 3: Is light matter?

Light helps us see, but let’s test it.

1. Does light have mass like an object?
2. Does light take up space like a book or water does?

No. Light is not a physical object made of matter.

Answer: No, light is not matter.

Worked Example 4: A big balloon and a small rock

Which one might have more mass: a big balloon full of air or a small rock?

The balloon is bigger in size, but the small rock may have more mass.

This teaches us that bigger does not always mean more mass.

Answer: Size and mass are different. A small object can have more mass than a larger object.

Try thinking like a scientist

When you look at something, ask:

• Is it a physical object or substance?
• Does it have mass?
• Does it take up space?

If the answer is yes, then it is matter.

Important ideas to remember

• Matter is anything that has mass and takes up space.
• Mass means how much matter is in an object.
• Space means the room something takes up.
• Air is matter, even though we cannot usually see it.
• Light and sound are not matter.
• Big objects do not always have more mass than small objects.

Brief Summary

Matter is all around us. Anything that has mass and takes up space is matter.

Mass tells us how much matter is in something. From rocks to water to air, matter can look very different, but it always has mass and takes up space.

Solids: Fixed Shape and Volume

Everything around us is made of matter. Matter is anything that takes up space. A chair, a rock, a pencil, and an ice cube are all matter.

One kind of matter is called a solid. In this lesson, you will learn why solids keep their own shape and why they take up the same amount of space unless something changes them.

What is a solid?

A solid is a kind of matter that has a fixed shape and a fixed volume.

• Fixed shape means it keeps its own shape.
• Fixed volume means it keeps the same amount of space it takes up.

For example, a book stays shaped like a book. A toy block stays shaped like a block. If you put them on a desk, in a box, or on the floor, they do not change into the shape of the container.

Why do solids have a fixed shape and volume?

Solids are made of tiny particles. These particles are packed very close together. They do not move around freely like particles in a liquid or gas.

Instead, the particles in a solid vibrate in place. That means they wiggle a little, but they stay in almost the same spot.

Because the particles are so close together, the solid keeps its shape. Because the particles are not spreading far apart, the solid also keeps the same volume.

You can think of it like children standing shoulder to shoulder in a line. They may wiggle a little, but they stay in place. That is similar to how particles in a solid act.

Shape and volume

Let’s look more closely at these two important ideas.

1. Fixed shape

   A solid has its own shape. A crayon is shaped like a crayon. A spoon is shaped like a spoon. If you move them somewhere else, their shape stays the same.

2. Fixed volume

   A solid takes up a certain amount of space. A marble takes up a small amount of space. A desk takes up more space. Each solid keeps its own amount of space unless it is cut, broken, melted, or changed in some other way.

Examples of solids

• a rock
• a pencil
• a coin
• a chair
• an eraser
• an ice cube

All of these objects keep their own shape and volume.

What happens when you move a solid?

If you carry a solid from one place to another, it is still the same solid. A block on a shelf and the same block in a backpack still has the same shape and volume.

The place changes, but the solid’s shape and volume do not.

Can a solid change?

Yes, a solid can change if something happens to it. For example, you can break a cracker into pieces. You can cut paper. You can melt an ice cube if it gets warm.

But if nothing changes the solid, it keeps its shape and volume.

Worked Example 1: Finding the solid

Question: Which of these is a solid: apple juice, air, or a wooden ruler?

Think: A solid has a fixed shape and a fixed volume.

Answer: The wooden ruler is a solid.

Why? The ruler keeps its own shape and takes up the same amount of space. Apple juice takes the shape of its cup, and air spreads out.

Worked Example 2: Does the shape change?

Question: Mia puts a toy car in a box. Does the toy car change shape to match the box?

Think: Solids keep their own shape.

Answer: No, the toy car does not change shape to match the box.

Why? The toy car is a solid. It has a fixed shape, so it stays shaped like a toy car.

Worked Example 3: Comparing two objects

Question: Which object is a better example of a solid: a basketball or milk?

Think: A solid keeps its shape and volume.

Answer: The basketball is the better example of a solid.

Why? A basketball keeps its shape. Milk does not keep its own shape. Milk takes the shape of the cup or bottle it is in.

Worked Example 4: Understanding particles

Question: In a solid, are particles far apart and moving everywhere, or are they close together and vibrating in place?

Answer: In a solid, the particles are close together and vibrating in place.

Why? This is why solids keep a fixed shape and fixed volume.

Try to remember

• Solids are a state of matter.
• Solids have a fixed shape.
• Solids have a fixed volume.
• The particles in a solid are tightly packed.
• The particles vibrate in place.

Quick check

1. Does a rock have a fixed shape?
2. Does water keep its own shape like a solid?
3. Are particles in a solid close together or far apart?
4. If you put a spoon in a drawer, does it change shape?

Answers to the quick check

1. Yes.
2. No.
3. Close together.
4. No.

Summary

A solid is matter with a fixed shape and a fixed volume. Its particles are packed closely together and vibrate in place. That is why a solid keeps its own shape and takes up the same amount of space.

Liquids: Flow and Adaptable Shape

Everything around us is made of matter. Matter can be a solid, a liquid, or a gas. In this lesson, we will learn about liquids.

A liquid is a kind of matter that can flow. This means it can move smoothly from one place to another. Water, milk, juice, and syrup are all liquids.

Liquids do not keep their own shape the way solids do. Instead, a liquid takes the shape of its container. If you pour water into a cup, it looks cup-shaped. If you pour the same water into a bowl, it looks bowl-shaped.

But even though a liquid changes shape, it still has an important property: it keeps the same amount. The liquid may look taller, shorter, wider, or flatter in different containers, but it is still the same liquid.

Why can liquids do this? Liquids are made of tiny pieces called particles. In a liquid, the particles are close together, but they are not stuck in one place. They have enough energy to slide past one another.

Because the particles can slide, the liquid can flow. Because the particles can move around, the liquid can change shape to fit its container.

Think about marbles in a box compared with water in a bottle. If marbles are packed tightly, they mostly stay where they are unless you move the whole box. But water can move and settle into the bottom of the bottle because its particles can slide around.

Main Ideas About Liquids

• Liquids can flow.
• Liquids do not have their own fixed shape.
• Liquids take the shape of their container.
• Liquid particles are close together.
• Liquid particles can slide past one another.

How Liquids Are Different From Solids

A solid, like a rock or a block, keeps its own shape. If you put a toy block in a basket, it is still a block shape.

A liquid is different. If you pour water into a glass, it becomes the shape of the glass. If you pour it into a bottle, it becomes the shape of the bottle.

This is one big way to tell solids and liquids apart:

• Solid: keeps its own shape
• Liquid: changes shape to fit the container

How Liquids Move

Liquids can be poured. They can drip. They can spread out. These are all ways liquids flow.

Some liquids flow very quickly, like water. Some liquids flow more slowly, like honey or syrup. They are all still liquids because they can move and take the shape of their container.

What Happens in Different Containers

Imagine you have the same amount of juice.

• In a tall, thin glass, the juice may look high.
• In a short, wide bowl, the juice may look low.

The shape changes because the container changes. The amount of juice stays the same.

You can think of it like this: the liquid spreads out to match the space inside the container.

Liquids and Particle Motion

We cannot usually see liquid particles with our eyes, but we can learn how they act. The particles in a liquid are always moving.

They have more kinetic energy than particles in many solids. That extra movement helps them slide past one another. You do not need to memorize the big words. Just remember: liquid particles move enough to slide.

When particles can slide, the whole liquid can move, pour, and change shape.

Everyday Examples of Liquids

• Water in a cup
• Milk in a carton
• Soup in a bowl
• Shampoo in a bottle
• Rain flowing in a puddle

In each example, the liquid fits the container or space it is in.

Worked Example 1

Question: Mia pours water from a round glass into a square container. What happens to the water?

Step 1: Remember that liquids do not keep their own shape.

Step 2: Remember that liquids take the shape of their container.

Answer: The water changes shape and fits the square container.

Worked Example 2

Question: Ben says, “Juice is in a tall cup, so it is a tall liquid.” Is Ben correct?

Step 1: Think about whether the liquid has its own shape.

Step 2: Liquids do not have a fixed shape. They take the shape of the container.

Answer: Ben is not correct. The juice is not naturally “tall.” It only looks tall because the cup is tall.

Worked Example 3

Question: A student pours the same amount of milk into two different containers. In one container, the milk level is high. In the other, the milk level is low and spread out. Is it possible that both containers hold the same amount of milk?

Step 1: Liquids can look different in different containers.

Step 2: A narrow container makes the liquid look higher. A wide container makes it spread out.

Answer: Yes. Both containers can have the same amount of milk even if the milk looks different.

Worked Example 4

Question: Why can syrup pour out of a bottle, even though it moves slowly?

Step 1: Ask if syrup can flow.

Step 2: Syrup does flow, even if it is slower than water.

Step 3: Its particles can still slide past one another.

Answer: Syrup is a liquid because it can flow and take the shape of its container, even though it flows slowly.

Try to Notice This Around You

• When you pour a drink, watch how it moves.
• When rainwater gathers, notice how it spreads into puddles.
• When soup is in a bowl, see how it matches the shape of the bowl.

These are clues that you are looking at a liquid.

Quick Check

1. Does a liquid keep its own shape?
2. What does a liquid do when you pour it into a new container?
3. Why can a liquid flow?
4. Can a slow-moving liquid still be a liquid?

Answers to the Quick Check

1. No, a liquid does not keep its own shape.
2. It takes the shape of the new container.
3. It can flow because its particles can slide past one another.
4. Yes, a slow-moving liquid is still a liquid.

Lesson Summary

Liquids are a state of matter that can flow. They do not have a fixed shape, so they take the shape of their container.

This happens because liquid particles are close together but can still move and slide past one another. That is why water, milk, juice, and syrup can all be poured and can fit different containers.

Gases: Expansion and Compressibility

Everything around us is made of matter. Matter can be a solid, a liquid, or a gas.

In this lesson, we will learn about gases. We will focus on two important ideas: expansion and compressibility.

Expansion means something spreads out to fill more space. Compressibility means something can be pushed into a smaller space.

Air is a gas. Even though we usually cannot see air, it is still matter. It takes up space and can move.

What is a gas like?

Gas is made of tiny pieces called particles. For 3rd grade, you can think of particles as tiny bits that are too small to see.

Gas particles move quickly and freely. They do not stay in one place. They spread out in all directions.

Because gas particles move around so much, a gas does not have its own shape. It takes the shape of its container.

A gas also does not have a fixed size. It can spread out to fill the space it is in.

Gas expansion

When a gas is in a container, it spreads out to fill the whole container. This is called expansion.

If you blow air into a balloon, the air spreads out inside the balloon. The balloon gets bigger because the gas fills the space inside it.

If the balloon is bigger, the gas spreads out more. The gas does not stay packed in one tiny corner.

Here are some examples of gas expansion:

• Air fills a balloon.
• Air fills a basketball.
• The smell of popcorn spreads through a room because gas particles move through the air.

Gas compressibility

Gas particles have spaces between them. Because of these spaces, gases can be compressed, or squeezed into a smaller space.

Think about a syringe without a needle. If air is inside and you push the plunger, the air can be squeezed smaller. That is compressibility.

Liquids and solids are not easy to squeeze smaller, but gases are. That is one special property of gases.

Why can gases expand and compress?

Gas particles are spread out. They move around with lots of space between them.

Because they are spread out, they can:

• Expand and fill a container
• Compress and fit into a smaller space

You can imagine gas particles like kids running around a playground. They are moving in many directions and are not packed tightly together.

How gases are different from solids and liquids

• Solids keep their own shape, like a book or a rock.
• Liquids flow and take the shape of their container, but they do not spread out to fill all the space.
• Gases take the shape of their container and spread out to fill all the space.

Gases are also much easier to squeeze than solids and liquids.

Worked Example 1: Balloon

Mia blows air into a balloon. What happens to the gas inside?

1. Air goes into the balloon.
2. The gas particles move around inside.
3. The gas spreads out to fill the balloon.
4. The balloon gets larger.

Answer: The gas expands to fill the balloon.

Worked Example 2: Pushing air into a smaller space

Leo has a syringe with air inside. He pushes the plunger down. What happens to the air?

1. The air particles are pushed closer together.
2. The air takes up less space.
3. This shows that gas can be squeezed.

Answer: The air is compressed. This shows compressibility.

Worked Example 3: Jar and room

A small amount of gas is let out of a jar into a room. Will the gas stay near the jar or spread through the room?

1. Gas particles move freely.
2. They spread out in open space.
3. The gas fills the space it can move into.

Answer: The gas will spread through the room. Gases expand to fill available space.

Worked Example 4: Choosing the best answer

Which sentence is true?

• A. Gas always stays in one corner.
• B. Gas can be squeezed into a smaller space.
• C. Gas has a fixed shape like a rock.

Step-by-step thinking:

1. Gas particles move around, so gas does not stay in one corner.
2. Gas does not have a fixed shape.
3. Gas can be squeezed because there is space between particles.

Answer: B. Gas can be squeezed into a smaller space.

Easy ways to remember

• Expand = spread out
• Compress = squeeze smaller
• Gas particles move fast and have space between them.

Try thinking about these everyday objects:

• A balloon gets bigger when filled with air.
• A bike tire holds compressed air.
• A pump pushes air into a ball.

All of these examples show that gases can fill spaces and can also be squeezed.

Quick check

• Does a gas have its own shape? No.
• Can a gas fill a container? Yes.
• Can a gas be squeezed into a smaller space? Yes.

Summary

Gases are made of tiny particles that move quickly and freely. Because the particles are spread out, gases can expand to fill a container and can be compressed into a smaller space.

Air in a balloon, air in a tire, and smells moving through a room are all examples of how gases behave. Remember: gases spread out and gases can be squeezed.

Melting and Freezing Points

Everything around us is made of matter. Matter can be a solid, liquid, or gas. In this lesson, we will focus on solids and liquids.

Sometimes matter changes from a solid to a liquid. Sometimes it changes from a liquid to a solid. These changes happen when matter gets warmer or cooler.

The temperature where a solid changes into a liquid is called the melting point. The temperature where a liquid changes into a solid is called the freezing point.

For many materials, the melting point and freezing point are the same temperature. Water is a great example. Water melts and freezes at $$0^\circ\text{C}$$.

What is melting?

Melting happens when a solid gets warm enough to turn into a liquid. Think about an ice cube on a table. As it warms up, it begins to melt into liquid water.

Ice does not melt at just any temperature. Ice melts at its melting point, which is $$0^\circ\text{C}$$. That means when ice reaches this temperature, it can change from solid ice to liquid water.

What is freezing?

Freezing happens when a liquid gets cool enough to turn into a solid. Think about water in an ice tray in a freezer. When it gets cold enough, the water freezes into ice.

Water freezes at $$0^\circ\text{C}$$. That is the freezing point of water.

Why do melting and freezing happen?

When matter gets warmer, it gains energy. This can help a solid loosen up and become a liquid.

When matter gets cooler, it loses energy. This can help a liquid slow down and become a solid.

You do not need to see the tiny parts of matter to know this is happening. You can notice the change by watching the material and checking its temperature.

Different materials have different melting and freezing points

Not all substances melt and freeze at the same temperature. Different kinds of matter have their own special melting and freezing points.

• Water melts and freezes at $$0^\circ\text{C}$$.
• Chocolate melts when it gets warm, which is why it can get soft in your hand or on a hot day.
• Butter melts when heated in a pan.
• Candle wax melts when a candle burns and cools back into a solid later.

This means each substance can change state at a precise, or exact, temperature.

How to tell if something will melt or freeze

You can compare the temperature of the material to its melting or freezing point.

• If a solid is warmed to its melting point, it can melt.
• If a liquid is cooled to its freezing point, it can freeze.
• If the temperature is not at that point yet, the material may stay in the same state.

For water:

• Above $$0^\circ\text{C}$$, water is usually a liquid.
• At $$0^\circ\text{C}$$, water can melt or freeze.
• Below $$0^\circ\text{C}$$, water is usually a solid, like ice.

Examples from everyday life

• An ice pop melts outside on a sunny day.
• A puddle can freeze on a very cold night.
• Butter melts on warm toast.
• Melted wax hardens when it cools.

Worked Example 1

Question: An ice cube is sitting in a warm room. Is it melting or freezing?

Step 1: An ice cube is a solid.

Step 2: The room is warm, so the ice cube is getting warmer.

Step 3: When a solid gets warm enough, it changes into a liquid. That is called melting.

Answer: The ice cube is melting.

Worked Example 2

Question: A tray of liquid water is placed in a freezer. Is the water melting or freezing?

Step 1: The water starts as a liquid.

Step 2: The freezer makes the water colder.

Step 3: When a liquid gets cold enough, it changes into a solid. That is called freezing.

Answer: The water is freezing.

Worked Example 3

Question: Water is at $$0^\circ\text{C}$$. What important temperature is this?

Step 1: Remember that water melts at $$0^\circ\text{C}$$.

Step 2: Remember that water also freezes at $$0^\circ\text{C}$$.

Answer: $$0^\circ\text{C}$$ is the melting point and freezing point of water.

Worked Example 4

Question: A student says, “All things melt and freeze at the same temperature.” Is that correct?

Step 1: Think about water, butter, chocolate, and wax.

Step 2: These materials do not all melt at the same temperature.

Step 3: Each substance has its own melting and freezing point.

Answer: No, that is not correct. Different materials have different melting and freezing points.

Important ideas to remember

1. Melting is when a solid changes to a liquid.
2. Freezing is when a liquid changes to a solid.
3. The melting point is the temperature where melting happens.
4. The freezing point is the temperature where freezing happens.
5. For water, both happen at $$0^\circ\text{C}$$.
6. Different substances have different melting and freezing points.

Brief Summary

Matter can change between solid and liquid when temperature changes. A solid melts at its melting point, and a liquid freezes at its freezing point. Water melts and freezes at $$0^\circ\text{C}$$. Different substances have their own exact temperatures for these changes.

Evaporation and Boiling

Have you ever seen a rain puddle disappear after a sunny day? Have you watched water bubble in a pot when it gets very hot? Both of these things happen when a liquid changes into a gas.

In this lesson, you will learn about evaporation and boiling. They are both ways a liquid can change into a gas, but they do not happen in the same way.

First, let’s remember the states of matter.

• Solid: keeps its shape, like ice.
• Liquid: flows and takes the shape of its container, like water.
• Gas: spreads out and fills space, like water vapor in the air.

When matter changes from one state to another, it is called a phase change. Evaporation and boiling are phase changes that change a liquid into a gas.

What is evaporation?

Evaporation is when a liquid changes into a gas slowly from its surface. The surface is the top part of the liquid that touches the air.

Evaporation can happen even when the liquid is not very hot. For example, a wet shirt hanging outside can dry. The water on the shirt slowly changes into gas and goes into the air.

During evaporation, only some tiny parts of the liquid near the top get enough energy to leave the liquid and become gas. That is why evaporation is usually slow.

Examples of evaporation:

• A puddle shrinking after rain
• Wet hair drying
• A spilled drink slowly disappearing
• Clothes drying on a clothesline

What helps evaporation happen faster?

• More heat: Warm water evaporates faster than cold water.
• Moving air: Wind can carry the gas away, so more liquid can evaporate.
• More surface area: Water spread out in a thin layer evaporates faster than water in a deep cup.

What is boiling?

Boiling is when a liquid changes into a gas quickly throughout the liquid, not just at the surface.

When water boils, you can see bubbles forming inside the liquid and rising to the top. These bubbles are filled with gas.

Boiling happens when a liquid gets hot enough. For water, the boiling point is about \(100\degree C\). That means water boils when it reaches about \(100\degree C\).

We can write that as:

$$\text{liquid water} \rightarrow \text{water vapor}$$

When boiling happens, the whole liquid is changing to gas, not only the top. That is why boiling is faster and easier to see than evaporation.

How are evaporation and boiling the same?

• Both change a liquid into a gas.
• Both need energy, usually heat.
• Both are changes in the state of matter.

How are evaporation and boiling different?

• Evaporation happens slowly. Boiling happens quickly.
• Evaporation happens only at the surface. Boiling happens throughout the liquid.
• Evaporation can happen at many temperatures. Boiling happens at a special hot temperature called the boiling point.
• Evaporation usually does not make bubbles. Boiling makes bubbles in the liquid.

Let’s think about what is happening.

Liquids are made of tiny parts that are always moving. When a liquid gets energy, these tiny parts move faster.

In evaporation, some tiny parts at the top move fast enough to escape into the air. In boiling, many tiny parts all through the liquid move fast enough to make bubbles of gas.

You do not need to see the tiny parts to understand the change. You can observe clues, like a puddle getting smaller or bubbles forming in hot water.

Worked Example 1: Drying Hands

Question: After washing your hands, you wave them in the air. Soon they feel dry. Is this evaporation or boiling?

Step 1: The water is leaving your skin slowly.

Step 2: The water is not bubbling.

Step 3: The change is happening at the surface of the water on your skin.

Answer: This is evaporation.

Worked Example 2: Pot of Water on a Stove

Question: A pot of water is heated on a stove. After a while, bubbles rise from all parts of the water. Is this evaporation or boiling?

Step 1: The water is very hot.

Step 2: Bubbles are forming inside the liquid.

Step 3: The liquid is changing to gas throughout the pot.

Answer: This is boiling.

Worked Example 3: Two Plates of Water

Question: Plate A has a thin layer of water. Cup B has the same amount of water, but it is deep. Which one will evaporate faster?

Step 1: Evaporation happens at the surface.

Step 2: The plate has more water spread out at the top.

Step 3: More surface area helps evaporation happen faster.

Answer: Plate A will evaporate faster.

Worked Example 4: Sunny Day or Windy Day?

Question: A puddle is outside. On which day might it disappear faster: a cool, still day or a warm, windy day?

Step 1: More heat helps evaporation.

Step 2: Moving air also helps evaporation.

Step 3: A warm, windy day has both heat and moving air.

Answer: The puddle will likely disappear faster on a warm, windy day.

Watch out for this common mix-up:

• If a liquid is disappearing slowly from the top, it is probably evaporation.
• If a liquid is bubbling and changing quickly, it is probably boiling.

Quick Check

1. When a puddle disappears after rain, is that evaporation or boiling?
2. Does boiling happen only at the surface or throughout the liquid?
3. Which usually has bubbles: evaporation or boiling?
4. Name one thing that can make evaporation faster.

Answers:

1. Evaporation
2. Throughout the liquid
3. Boiling
4. More heat, moving air, or more surface area

Summary

Evaporation and boiling both change a liquid into a gas. Evaporation is a slow change that happens at the surface of a liquid. Boiling is a fast change that happens throughout the liquid and makes bubbles.

When you see wet things dry, you are seeing evaporation. When you see water bubbling in a hot pot, you are seeing boiling. Knowing the difference helps you understand how heat can change matter.

Condensation and Precipitation

Have you ever seen water drops on the outside of a cold cup? Have you looked up and seen clouds in the sky, then later watched rain fall? These things happen because of condensation and precipitation.

In this lesson, you will learn what condensation is, what precipitation is, and how they are connected. You will also learn how cooling changes water in the air.

What is matter?

Matter is anything that takes up space. Water is matter. Water can be found in different states:

• Solid like ice
• Liquid like water you drink
• Gas like water vapor in the air

Water can change from one state to another when it gains or loses heat.

What is condensation?

Condensation happens when a gas cools and changes into a liquid.

Water vapor is a gas. It is in the air all around us, even when we cannot see it. When water vapor cools down, its tiny particles slow down and move closer together. Then liquid water forms.

This change can be shown like this:

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

That is condensation.

How does cooling cause condensation?

When water vapor is warm, the particles move around more and stay spread out. When the water vapor cools, the particles slow down.

As the particles slow down, they come closer together. Soon, they form tiny drops of liquid water. Those tiny drops are condensation.

Examples of condensation

• Water drops form on the outside of a cold glass.
• A bathroom mirror gets foggy after a hot shower.
• Dew forms on grass in the morning.
• Clouds form in the sky.

In each example, water vapor in the air cools and turns into liquid water.

What is precipitation?

Precipitation is water that falls from clouds to Earth.

Precipitation can be:

• Rain
• Snow
• Sleet
• Hail

For 3rd grade, it is most important to know that precipitation happens after water collects in clouds. When the drops or pieces get heavy enough, they fall to the ground.

How are condensation and precipitation connected?

Condensation helps make clouds. First, water vapor in the air cools. Then it condenses into tiny drops of liquid water. These tiny drops gather together in clouds.

As more and more water gathers, the drops can become too heavy to stay in the cloud. Then they fall as precipitation, such as rain.

So the order is:

1. Water vapor is in the air.
2. The water vapor cools.
3. Condensation happens.
4. Tiny water drops form clouds.
5. The drops get bigger and heavier.
6. Precipitation falls to Earth.

Condensation is not the same as precipitation

It is easy to mix these up, so let us make it clear.

• Condensation means gas changes into liquid.
• Precipitation means water falls from clouds to Earth.

Condensation often happens before precipitation.

Worked Example 1: The cold cup

You put ice water in a cup. Soon, water drops appear on the outside of the cup. Where did the drops come from?

Step 1: The cup is cold.

Step 2: Water vapor in the air touches the cold cup.

Step 3: The water vapor cools down.

Step 4: It changes from a gas to a liquid.

Answer: The drops came from condensation. They came from water vapor in the air, not from leaking through the cup.

Worked Example 2: Foggy bathroom mirror

After a hot shower, the bathroom mirror looks foggy. Why?

Step 1: The shower puts lots of water vapor into the air.

Step 2: The mirror is cooler than the warm air.

Step 3: Water vapor touches the cool mirror and cools down.

Step 4: Tiny liquid drops form on the mirror.

Answer: This is condensation.

Worked Example 3: Clouds and rain

The sun warms water on Earth. Some of the water becomes water vapor and goes into the air. Later, clouds form, and then it rains. What happened?

Step 1: Water vapor rose into the air.

Step 2: Higher in the sky, the air was cooler.

Step 3: The water vapor cooled and condensed into tiny drops.

Step 4: The tiny drops made clouds.

Step 5: The drops joined together and got heavy.

Step 6: They fell as rain.

Answer: Condensation formed the cloud, and precipitation was the rain that fell.

Worked Example 4: Choose the correct word

A cloud forms in the sky. Later, snow falls to the ground.

Which word matches each part?

• Cloud forming = ?
• Snow falling = ?

Step 1: Cloud forming happens when water vapor cools and turns into tiny liquid drops. That is condensation.

Step 2: Snow falling from the cloud to the ground is precipitation.

Answer:

• Cloud forming = condensation
• Snow falling = precipitation

Helpful clues to remember

• If water vapor cools and becomes liquid, think condensation.
• If water falls from clouds, think precipitation.
• Clouds form because of condensation.
• Rain, snow, sleet, and hail are kinds of precipitation.

Quick check

• What state of matter is water vapor? Gas
• What happens to particles when a gas cools? They slow down and move closer together.
• What is the change from gas to liquid called? Condensation
• What is water that falls from clouds called? Precipitation

Summary

Condensation happens when water vapor cools and changes from a gas into a liquid. The particles slow down and come closer together, forming tiny drops of water.

Precipitation happens when water falls from clouds to Earth as rain, snow, sleet, or hail. Condensation helps form clouds, and precipitation happens when water in clouds gets heavy enough to fall.

Reversible vs. Irreversible Changes

Everything around us is made of matter. Matter can change in different ways. Some changes can be undone, and some cannot.

In this lesson, you will learn the difference between reversible changes and irreversible changes. This will help you understand what happens when matter changes.

What is a reversible change?

A reversible change is a change that can go back to the way it was before. The material is still the same material, even if it looks different for a while.

Many reversible changes happen when matter changes state. A state of matter can be a solid, liquid, or gas.

• A solid can melt into a liquid.
• A liquid can freeze into a solid.
• A liquid can change into a gas.
• A gas can cool and become a liquid again.

These changes are reversible because the matter can change back.

Examples of reversible changes

• Ice melts into water. Water can freeze back into ice.
• Chocolate melts when it gets warm. It can turn solid again when it cools.
• Water in a puddle dries into water vapor. Later, water vapor can cool and become liquid water.

What is an irreversible change?

An irreversible change is a change that cannot easily be changed back. After the change, the material is different in an important way.

Irreversible changes often make a new material or change the object so much that it cannot return to what it was before.

Examples of irreversible changes

• Baking a cake
• Cooking an egg
• Burning paper
• Rust forming on metal
• Tearing paper

After these changes, you cannot easily get back the starting material.

How can we tell the difference?

Here is a good question to ask: Can this change go back?

If the answer is yes, it is probably a reversible change.

If the answer is no, it is probably an irreversible change.

You can also think about whether the material is still the same material.

• If it is still the same material, the change may be reversible.
• If a new material is made, the change is usually irreversible.

Reversible changes and heating or cooling

Heating and cooling often cause reversible changes. For example, heating ice makes it melt. Cooling water makes it freeze.

We can show this change like this:

ice \(\rightarrow\) water \(\rightarrow\) ice

The matter stays the same. It is still water, just in different states.

Irreversible changes and permanent change

Some changes are permanent. That means they stay changed.

For example, when you bake bread, the dough changes into bread. You cannot turn the bread back into dough.

When wood burns, it turns into ash and smoke. You cannot turn the ash back into the same piece of wood.

Worked Example 1

Question: An ice cube melts in the sun. Is this reversible or irreversible?

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

Step 2: Can it change back? Yes. Water can freeze and become ice again.

Answer: This is a reversible change.

Worked Example 2

Question: A piece of paper is burned. Is this reversible or irreversible?

Step 1: What happened? The paper changed into ash and smoke.

Step 2: Can it change back into the same paper? No.

Answer: This is an irreversible change.

Worked Example 3

Question: Butter melts on warm toast. Is this reversible or irreversible?

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

Step 2: Can it change back? Yes. If it cools, it can become solid butter again.

Answer: This is a reversible change.

Worked Example 4

Question: An egg is cooked in a pan. Is this reversible or irreversible?

Step 1: What happened? Heat changed the egg.

Step 2: Can the cooked egg become a raw egg again? No.

Answer: This is an irreversible change.

Let’s compare them

• Reversible change: can be changed back
• Irreversible change: cannot be changed back easily
• Reversible change: often a change of state
• Irreversible change: often makes a new material or causes a permanent change

Helpful clue words

• Reversible: melt, freeze, cool, warm, change of state
• Irreversible: burn, cook, bake, rust, tear

Try thinking about these

1. Water freezes into ice.
2. A cookie is baked in the oven.
3. A snowman melts.
4. A log burns in a fire.

The answers are:

1. Reversible
2. Irreversible
3. Reversible
4. Irreversible

Summary

A reversible change can be undone. The matter stays the same, even if it changes state.

An irreversible change cannot be undone easily. It often makes a new material or causes a permanent change.

When you are not sure, ask yourself: Can it change back? That question can help you decide.

Physical Changes in Matter

Everything around us is made of matter. Matter is anything that takes up space and has weight. A book, a puddle, a balloon, and even the air in a room are all matter.

Matter can change in different ways. Today we are learning about a physical change. A physical change happens when matter changes its size, shape, or state, but it is still the same kind of matter.

For example, if you tear paper into smaller pieces, it is still paper. If an ice cube melts, it is still water. The matter may look different, but it has not turned into a new kind of substance.

What can change in a physical change?

• Size: Something can be cut, broken, or crushed into smaller pieces.
• Shape: Something can be bent, stretched, folded, or squished.
• State: Matter can change from solid to liquid or liquid to gas.

What stays the same?

In a physical change, the matter stays the same substance. Its basic identity does not change. Ice, liquid water, and water vapor are all water.

States of matter are the main forms matter can be in:

• Solid: keeps its own shape, like a rock or an ice cube
• Liquid: flows and takes the shape of its container, like milk or water
• Gas: spreads out to fill the space around it, like air or steam

When matter changes from one state to another, that can be a physical change.

• Melting: solid to liquid
• Freezing: liquid to solid
• Evaporating: liquid to gas
• Condensing: gas to liquid

These changes often happen when matter gains or loses heat energy. But even when the state changes, the substance can still be the same substance.

Examples of physical changes

• Cutting an apple into slices
• Tearing paper
• Crushing a can
• Bending a straw
• Melting butter
• Freezing juice into a popsicle
• Water evaporating from a puddle

In all of these examples, the matter changes how it looks, feels, or exists, but it does not become a new kind of matter.

How can you tell if something is a physical change?

Ask yourself these questions:

1. Did the matter only change size, shape, or state?
2. Is it still the same substance as before?
3. Can I describe what changed without saying it became something new?

If the answer is yes, it is probably a physical change.

Worked Example 1: Tearing paper

A student tears one sheet of paper into 4 small pieces.

What changed? The size and shape changed.

What stayed the same? It is still paper.

Answer: This is a physical change.

We can even think about the number of pieces:

$$1\text{ sheet} \rightarrow 4\text{ pieces}$$

Even though the number of pieces changed, the substance is still paper.

Worked Example 2: Melting an ice cube

An ice cube sits on a plate and melts into liquid water.

What changed? The state changed from solid to liquid.

What stayed the same? It is still water.

Answer: This is a physical change.

We can show the change like this:

$$\text{ice} \rightarrow \text{water}$$

Ice and water may look different, but they are the same substance.

Worked Example 3: Crushing a soda can

A can is stepped on and becomes flat.

What changed? The shape changed.

What stayed the same? It is still made of the same metal.

Answer: This is a physical change.

The can looks different, but it did not turn into a new substance.

Worked Example 4: Freezing juice

Orange juice is poured into a mold and placed in a freezer. Later, it becomes a frozen juice pop.

What changed? The state changed from liquid to solid.

What stayed the same? It is still orange juice.

Answer: This is a physical change.

The juice changed form, but it is still the same kind of matter.

Let’s compare

Here are signs of a physical change:

• The matter is cut, broken, bent, or crushed.
• The matter melts, freezes, evaporates, or condenses.
• The substance stays the same.

Important idea: Looking different does not always mean being different. Matter can look new on the outside but still be the same substance on the inside.

Try thinking about these

• If you break a crayon in half, is it still crayon? Yes.
• If water turns to ice, is it still water? Yes.
• If you fold a towel, is it still a towel? Yes.

These are all physical changes because the matter stays the same even though its size, shape, or state changes.

Summary

A physical change is a change in matter that affects its size, shape, or state. The most important thing to remember is that the substance stays the same. Tearing, cutting, bending, melting, freezing, evaporating, and condensing are all examples of physical changes.

Mixtures and Pure Substances

Everything around us is made of matter. Matter is anything that takes up space and has mass. Some kinds of matter are made of only one material, and some are made by putting different materials together.

In this lesson, we will learn about pure substances and mixtures. We will practice how to tell the difference between them by looking carefully at what something is made of.

What is a pure substance?

A pure substance is made of just one kind of material. It is the same all the way through.

For example, if you have a glass of clean water, it is one kind of material: water. If you have a piece of gold, it is one kind of material: gold. These are pure substances.

A pure substance does not have different parts mixed together. If you look at it, every part is the same kind of matter.

What is a mixture?

A mixture is made when two or more materials are put together. In a mixture, each material keeps its own properties.

For example, trail mix is a mixture. It may have raisins, cereal, nuts, and chocolate pieces. Each part is still its own thing, even though they are together in one bowl.

Salad is also a mixture. Lettuce, tomatoes, carrots, and cucumbers are mixed together, but each food is still different.

How are pure substances and mixtures different?

• Pure substance: only one material
• Mixture: two or more materials together
• Pure substance: the same all the way through
• Mixture: may have different parts you can see or different materials spread together

Think of it this way: if something is made of only one kind of matter, it is a pure substance. If it is made by combining different kinds of matter, it is a mixture.

Can you always see the different parts?

No. In some mixtures, you can easily see the parts. In a fruit salad, you can point to each fruit piece.

But in other mixtures, the parts are too small to see separately. For example, when salt is stirred into water, the salt seems to disappear. It is still there, so salt water is a mixture, not a pure substance.

Examples of pure substances

• Clean water
• Gold
• Table salt by itself
• Sugar by itself

Each of these is just one material.

Examples of mixtures

• Trail mix
• Salad
• Salt water
• Cereal with milk

Each of these has two or more materials together.

How can we tell if something is a mixture or a pure substance?

Ask these questions:

1. Is it made of only one material?
2. Or is it made of two or more materials put together?
3. Do the parts stay their own kind of matter?

If it is only one material, it is a pure substance. If it has two or more materials together, it is a mixture.

Worked Example 1: Apple juice or fruit salad?

Let us compare two things.

Fruit salad: It has apples, grapes, bananas, and strawberries together. That means it has more than one material.

Answer: Fruit salad is a mixture.

Apple juice: If we think of it as just one drink made of apple juice, we treat it as one material in this lesson.

Answer: Apple juice is a pure substance for this grade-level idea.

Worked Example 2: Salt or salt water?

Salt: By itself, salt is one material.

Answer: Salt is a pure substance.

Salt water: Salt and water are put together. That makes two materials.

Answer: Salt water is a mixture.

Worked Example 3: Cereal in a box

A cereal box may have flakes, marshmallows, or oat pieces. If there is more than one kind of part together, it is not just one material.

Answer: The cereal is a mixture.

If the box had only one kind of cereal piece and nothing else, then we could think of it as one material for this lesson.

Worked Example 4: A bowl with water, sand, and small rocks

Let us count the materials:

• Water
• Sand
• Small rocks

There are 3 materials. We can show that with math: \(1 + 1 + 1 = 3\).

Because there are different materials together, the bowl holds a mixture.

Why does this matter?

Knowing about mixtures and pure substances helps us describe the world. It helps us sort materials and understand what things are made of.

Scientists study materials carefully. One way they do that is by asking whether a sample is one material or a combination of materials.

Try to classify these on your own

• Clean water
• Lemonade
• A handful of coins and paper clips
• Sugar
• Soup with noodles and vegetables

Possible answers:

• Clean water: pure substance
• Lemonade: mixture
• Coins and paper clips: mixture
• Sugar: pure substance
• Soup with noodles and vegetables: mixture

Important idea to remember

A mixture can often be made by physically combining materials. That means the materials are put together, but they do not turn into a brand-new material for this lesson. They are still the same materials, just side by side or stirred together.

A pure substance is not made by combining different materials. It is just one material all through.

Summary

A pure substance is made of one kind of material. A mixture is made of two or more materials together.

Some mixtures have parts you can see, like salad. Some mixtures do not show their parts easily, like salt water. To tell the difference, ask: is it one material, or is it made of different materials together?

Solutions and Solubility

Have you ever stirred sugar into water and then watched the sugar seem to disappear? It did not really vanish. It mixed into the water so well that it spread out evenly. This is called a solution.

In science, a solution is a special kind of mixture. It looks the same all the way through. You cannot easily see the parts after they are mixed.

This lesson will help you learn what a solution is, what solute and solvent mean, and what solubility means.

What Is a Solution?

A solution is a mixture where one material dissolves evenly into another material.

• The material that gets dissolved is called the solute.
• The material that does the dissolving is called the solvent.

When the solute and solvent mix evenly, they make one smooth mixture called a solution.

Example: If you put salt into water and stir, the salt is the solute and the water is the solvent.

What Does Dissolve Mean?

To dissolve means to break up and spread out evenly in another material. When something dissolves, it may look like it disappeared, but it is still there.

You can often tell it is still there by tasting it, smelling it, or seeing what happens after the water dries up.

What Is Solubility?

Solubility means how well something can dissolve in a liquid.

Some materials dissolve very well. Some dissolve only a little. Some do not dissolve at all.

• Sugar in water: dissolves well
• Salt in water: dissolves well
• Sand in water: does not dissolve
• Oil in water: does not mix evenly into a solution

How Can You Tell If Something Is a Solution?

A mixture is a solution if:

• it looks the same all the way through,
• the solute is spread evenly, and
• you cannot easily separate the parts just by looking.

If you can still see big pieces, clumps, or layers, it is probably not a solution.

Examples of Solutions in Everyday Life

• Salt water
• Sugar water
• Hot chocolate mix stirred well into milk or water
• Drink mix stirred into water

Examples That Are Not Solutions

• Sand in water
• Oil and water
• Muddy water

These are mixtures, but they are not solutions because they do not mix evenly.

What Helps Something Dissolve?

Some things can help a solute dissolve faster or better.

• Stirring: Mixing helps the solute spread out.
• Warm liquid: Many solids dissolve faster in warmer water.
• Time: Some solutes need more time to dissolve.

Even if stirring helps, not everything will dissolve. Sand will still not dissolve well in water.

There Is a Limit

A solvent can only dissolve so much solute. After a while, no more will dissolve.

For example, if you keep adding spoonfuls of sugar to a cup of water, at first the sugar dissolves. But if you add too much, some sugar will stay at the bottom.

That means the water cannot dissolve any more sugar.

Worked Example 1: Finding the Solute and Solvent

Mia puts a spoonful of salt into a glass of water and stirs.

1. What is being dissolved? The salt.
2. What is doing the dissolving? The water.
3. What is the mixture called after the salt spreads out evenly? A solution.

Answer: The solute is salt, the solvent is water, and together they make a solution.

Worked Example 2: Is It a Solution?

Leo mixes sand and water in a cup. He stirs it. After a little while, the sand sinks to the bottom.

1. Did the sand dissolve evenly? No.
2. Does the mixture look the same all the way through? No.
3. Is it a solution? No.

Answer: Sand and water are not a solution because the sand does not dissolve.

Worked Example 3: Which One Dissolves?

Sara has three things: sugar, pebbles, and salt. She puts each one into water.

• Sugar mixes in and cannot be seen after stirring.
• Pebbles stay solid at the bottom.
• Salt mixes in and cannot be seen after stirring.

Step-by-step thinking:

1. If something dissolves evenly, it can make a solution.
2. Sugar dissolves, so sugar and water make a solution.
3. Pebbles do not dissolve, so pebbles and water do not make a solution.
4. Salt dissolves, so salt and water make a solution.

Answer: Sugar and salt dissolve in water. Pebbles do not.

Worked Example 4: Too Much Solute

Ben puts drink mix into water. He stirs and it dissolves. Then he keeps adding more and more drink mix. Soon, some powder stays at the bottom.

1. At first, the drink mix dissolves, so it makes a solution.
2. Later, extra powder stays at the bottom.
3. This means the water cannot dissolve any more.

Answer: The water reached its limit. Some of the solute could not dissolve.

Tips to Remember

• A solute gets dissolved.
• A solvent does the dissolving.
• A solution looks even all the way through.
• Solubility tells how well something dissolves.
• Not all mixtures are solutions.

Quick Check

1. If sugar dissolves in water, what is the solute?
2. If something stays in clumps and sinks, is it a solution?
3. What does solubility mean?
4. Name one thing that can help some solids dissolve faster.

Answers:

1. The sugar
2. No
3. How well something can dissolve in a liquid
4. Stirring, warmer liquid, or time

Summary

A solution is a mixture where a solute dissolves evenly in a solvent. The solute is the material being dissolved, and the solvent is the material that dissolves it.

Solubility tells how well a material dissolves. Sugar and salt dissolve well in water, but sand does not. When you understand solutions and solubility, you can tell which mixtures are smooth, even solutions and which are not.

Conductivity and Insulation are ways we describe how materials let energy move through them.

Some materials let thermal energy move through easily. Thermal energy is heat. Some materials also let electrical energy move through easily. Electrical energy is the energy in electricity.

A material that lets heat or electricity move through it easily is called a conductor. A material that does not let heat or electricity move through it easily is called an insulator.

Learning about conductors and insulators helps us choose the right materials for cooking, building, and staying safe around electricity.

Main Idea 1: Conductors let energy move.

Conductors are materials that allow heat or electricity to pass through them more easily than other materials.

Many metals are good conductors. For example, copper, aluminum, and steel can let energy move through them.

If you touch a metal spoon sitting in a hot bowl of soup, the spoon may get warm quickly. That is because the metal conducts thermal energy.

Wires are often made of metal because metal conducts electrical energy well. This helps electricity move from one place to another.

Main Idea 2: Insulators slow down or stop energy.

Insulators are materials that do not let heat or electricity move through them easily.

Materials like rubber, plastic, wood, foam, and cloth are often good insulators.

A pot may have a metal bottom to heat food, but the handle may be covered in plastic or rubber. The metal helps heat move into the food, and the plastic or rubber helps protect your hand from the heat.

Electrical cords often have metal on the inside and rubber or plastic on the outside. The metal carries electricity, and the outside covering helps keep people safe.

Main Idea 3: A material can be useful because of how it conducts or insulates.

We do not say one material is always better than another. A conductor is useful in some jobs, and an insulator is useful in other jobs.

• Conductors are helpful when we want heat or electricity to move.
• Insulators are helpful when we want to slow down heat or electricity.

For example, a frying pan is often metal because metal conducts heat. An oven mitt is made of thick fabric because fabric helps insulate your hand from heat.

Main Idea 4: We can classify materials.

To classify means to sort into groups by their properties. A property is something we can observe about a material.

When we classify materials by conductivity and insulation, we ask questions like these:

• Does this material let heat move through easily?
• Does this material let electricity move through easily?
• Would this material be safer as a cover or handle?

Then we can sort materials into groups such as:

• Good conductors
• Good insulators

Heat Conductors and Heat Insulators

Let us think first about heat.

If one end of a metal object gets hot, the heat can travel through the metal to the other end. That means metal is a heat conductor.

If one end of a wooden or plastic object gets warm, the heat usually moves more slowly. That means wood and plastic are better heat insulators.

This is why some cooking tools are made with two kinds of materials:

• Metal part: to heat food
• Plastic, wood, or rubber part: to protect hands

Electrical Conductors and Electrical Insulators

Now let us think about electricity.

Electricity needs a path to move. Some materials, especially metals, give electricity a good path. These materials are electrical conductors.

Rubber and plastic do not give electricity an easy path. These materials are electrical insulators.

That is why the outside of many cords is rubber or plastic. It helps keep the electricity inside the wire where it belongs.

Important Safety Note

Electricity can be dangerous. We should never test electrical materials by plugging things in or touching wires. Scientists and electricians use special tools and follow safety rules.

In class, we can learn about conductors and insulators by looking at objects and thinking about what materials they are made from.

Examples of Common Materials

• Metal: usually a conductor of heat and electricity
• Plastic: usually an insulator
• Rubber: usually an insulator
• Wood: often an insulator
• Cloth: often an insulator, especially for heat
• Foam: often an insulator

Worked Example 1: Sorting simple materials

Question: Is a metal spoon a conductor or an insulator?

Step 1: Think about what metal does. Metal lets heat move through it easily.

Step 2: Decide on the group. Since heat moves through metal, a metal spoon is a conductor.

Answer: A metal spoon is a conductor.

Worked Example 2: Choosing a safe handle

Question: A soup pot gets very hot. Which is better for the handle: metal or plastic?

Step 1: Think about what the handle needs to do. It should help protect your hand from heat.

Step 2: Compare the materials. Metal is a conductor, so it gets hot more easily. Plastic is an insulator, so heat moves through it less easily.

Step 3: Choose the safer material. Plastic is the better choice for the handle.

Answer: Plastic is better because it is an insulator.

Worked Example 3: Looking at an electrical cord

Question: Why does an electrical cord have metal inside and rubber outside?

Step 1: Think about the job of the inside part. Electricity needs to move, so the inside should be a conductor.

Step 2: Think about the job of the outside part. People may touch the outside, so it should help keep them safe.

Step 3: Match the materials to the jobs. Metal conducts electricity. Rubber insulates and helps protect people.

Answer: The cord has metal inside to carry electricity and rubber outside to help keep people safe.

Worked Example 4: Classifying several objects

Question: Put each object into the correct group: aluminum foil, oven mitt, wooden ruler, copper wire.

Step 1: Identify the materials.

• Aluminum foil is metal.
• Oven mitt is thick cloth.
• Wooden ruler is wood.
• Copper wire is metal.

Step 2: Sort by property.

• Metals are usually conductors.
• Cloth and wood are usually insulators.

Step 3: Make the groups.

Conductors: aluminum foil, copper wire

Insulators: oven mitt, wooden ruler

Answer: Aluminum foil and copper wire are conductors. Oven mitt and wooden ruler are insulators.

How to Remember the Difference

• Conductor = lets energy go through
• Insulator = slows down or blocks energy

You can think of it like this: a conductor is like an open road for energy, while an insulator is like a roadblock that makes energy move slowly or not at all.

Things to Watch Out For

• Do not assume every object is all one kind of material. Some objects have both conductors and insulators.
• Do not forget that heat and electricity are different, but both can move through materials.
• Do not think conductors are bad. Conductors are useful when we want energy to move.
• Do not think insulators are always better. Insulators are useful when we want protection.

Quick Check

1. A rubber glove is mostly a conductor or an insulator? Insulator.
2. Why are many pans made of metal? Because metal conducts heat well.
3. Why might a lunch box use foam inside? Because foam is an insulator and helps slow heat movement.
4. Is copper wire usually used to carry electricity or block it? Carry it.

Summary

Materials can be grouped by how they let energy move.

Conductors let heat or electricity move through easily. Metals are often conductors.

Insulators do not let heat or electricity move through easily. Rubber, plastic, wood, cloth, and foam are often insulators.

We use conductors when we want energy to move, and we use insulators when we want to stay safe or keep heat from moving too quickly.

Material Engineering: Choosing Materials for Purpose

Everything around us is made of matter. Matter is the “stuff” things are made of. A chair, a spoon, a raincoat, and a window are all made of different materials.

Engineers are people who design and build things to solve problems. When engineers make something, they must choose the best material for the job. This is called choosing materials for a purpose.

For example, a rain boot should keep water out. A cooking pot should be strong and able to handle heat. A window should be clear so light can pass through. Different jobs need different materials.

To choose the best material, engineers look at the material’s properties. Properties are the special ways a material looks, feels, or acts.

Some materials come from nature. These are called natural materials. Wood, cotton, wool, and stone are natural materials.

Some materials are made by people. These are called synthetic materials. Plastic and nylon are examples of synthetic materials.

Some materials are made by mixing two or more materials together to make something stronger or more useful. These are called composite materials. Plywood is one example because it is made from thin layers of wood pressed together.

Main Idea: Engineers do not pick materials at random. They study the properties of materials and match those properties to the job that needs to be done.

Important Properties of Materials

Here are some properties engineers think about when choosing materials:

• Strength — Can it hold heavy things without breaking?
• Hardness — Is it hard to scratch or dent?
• Flexibility — Can it bend without breaking?
• Water resistance — Does it keep water out?
• Thermal properties — Does it let heat move through it, or does it block heat?
• Chemical properties — Does it change when it touches air, water, or other substances?
• Transparency — Can you see through it?
• Weight — Is it light or heavy?

Structural Properties

Structural properties help us know how a material behaves when we push, pull, bend, or press on it.

A bridge needs strong materials because it must hold many cars and trucks. A pillow does not need to be very strong, but it should be soft and flexible.

Wood is often used for furniture because it is strong. Rubber is used for things like erasers and some playground surfaces because it is flexible and soft.

Thermal Properties

Thermal means heat. Some materials let heat move through them easily. Some do not.

Metal often gets hot quickly, so it is useful for pots and pans. That helps food cook. But a metal handle can get too hot to touch.

That is why some pot handles are made of plastic or wood. These materials do not let heat move through as easily, so they can help protect our hands.

Chemical Properties

Chemical properties tell us how a material may change when it touches other things.

For example, some metals can rust when they get wet and stay wet for a long time. Rust is a change that can make the metal weaker.

Engineers think about this when building outdoor objects like fences, bikes, or playground equipment. They may choose materials that do not rust easily, or they may cover the metal with paint to protect it.

Natural, Synthetic, and Composite Materials

Let’s look at the three big groups of materials again:

• Natural materials: come from plants, animals, or the Earth. Examples: wood, cotton, leather, stone.
• Synthetic materials: made by people. Examples: plastic, nylon, some foams.
• Composite materials: made by combining materials. Examples: plywood, some kinds of helmets, and some sports equipment.

Each kind of material can be useful. Engineers ask, “What does this object need to do?” Then they choose a material that has the right properties.

How Engineers Choose Materials

Engineers often think through questions like these:

1. What problem am I trying to solve?
2. What does the object need to do?
3. Should it be strong, soft, clear, waterproof, or heat-safe?
4. Will it be used indoors or outdoors?
5. Will it touch water, heat, or chemicals?
6. Which material has the best properties for this job?

Worked Example 1: Choosing a Material for a Raincoat

Problem: A raincoat needs to keep a person dry in the rain.

Think about the properties:

• It should be water-resistant.
• It should be lightweight.
• It should be a little flexible so the person can move.

Best choice: A synthetic material like plastic-coated fabric works well.

Why? Cotton feels nice, but it can soak up water. A raincoat must keep water out, so a water-resistant material is a better choice.

Worked Example 2: Choosing a Material for a Window

Problem: A window should let light in and allow people to see through it.

Think about the properties:

• It should be transparent, which means see-through.
• It should be hard.
• It should be able to stay strong in wind and weather.

Best choice: Glass is a good choice.

Why? Wood is strong, but you cannot see through it. Cloth is soft, but it does not work as a window. Glass is see-through and hard, so it fits the job.

Worked Example 3: Choosing a Material for a Cooking Spoon

Problem: A spoon used for stirring hot soup should work safely in a hot pot.

Think about the properties:

• It should be strong.
• It should not get too hot too quickly in your hand.
• It should be safe to use with hot food.

Best choice: Wood or certain kinds of heat-safe plastic can be good choices.

Why? A metal spoon is strong, but it may get hot fast. Wood does not let heat move as easily, so it can be safer to hold while stirring.

Worked Example 4: Choosing a Material for a Playground Slide

Problem: A playground slide needs to be strong, smooth, and safe outdoors.

Think about the properties:

• It should be strong enough to hold children.
• It should be smooth.
• It should handle sun and rain.

Possible choices: Metal or strong plastic.

Which is better? Engineers compare the choices. Metal is strong, but it can get very hot in the sun. Strong plastic can also be smooth and strong, and it may stay cooler than metal.

Conclusion: Engineers choose by thinking about more than one property at a time.

Sometimes One Material Is Not Enough

Some objects are made from more than one material because each part has a different job.

Think about an umbrella:

• The cover should be water-resistant.
• The pole should be strong.
• The handle should be easy to hold.

This shows that engineers may choose different materials for different parts of the same object.

Matching Materials to Jobs

Here are some common matches:

• Wood — furniture, some tools, building parts; strong and fairly easy to shape.
• Metal — pots, bikes, bridges; strong and often good with heat.
• Plastic — bottles, toys, containers; lightweight and often water-resistant.
• Glass — windows, jars; transparent and hard.
• Rubber — tires, gloves, seals; flexible and grippy.
• Fabric — clothes, bags, curtains; soft and flexible.

Be Careful: No Material Is Best for Everything

A material that is great for one job may be a poor choice for another job.

• Glass is great for windows, but not for a soccer ball.
• Rubber is great for an eraser, but not for a clear drinking cup.
• Metal is great for a pan, but not always for the handle.

That is why engineers compare properties before they decide.

Quick Check for Thinking

If you were choosing a material for each object, what property would matter most?

• A lunchbox: should be strong and easy to carry.
• A fish tank: should be waterproof and see-through.
• A winter hat: should help keep heat in.
• A soccer goal net: should be flexible and strong enough to stop the ball.

When you answer questions like these, always ask yourself: What does the object need to do?

Summary

Material engineering means choosing the right material for a certain job. Engineers study the properties of materials, such as strength, flexibility, heat behavior, and how they react to water or air.

Materials can be natural, synthetic, or composite. The best material depends on the purpose of the object. Smart engineers match the material’s properties to the job that needs to be done.