Changes in Matter

Physical Changes happen when matter changes its state, shape, size, or appearance, but it stays the same kind of substance.

For example, ice can melt into liquid water. The form changes, but it is still water. Cutting paper makes it smaller, but it is still paper. These are physical changes.

In this lesson, you will learn how to recognize physical changes, how thermal energy can cause them, and how they are different from changes that make a new substance.

What is a physical change?

A physical change is a change you can observe with your senses. A substance may look different after the change, but its tiny particles are still the same kind of matter as before.

Physical changes can affect:

• State — solid, liquid, or gas
• Shape — bent, stretched, folded, or broken
• Size — cut, crushed, or torn into smaller pieces
• Appearance — dissolved or mixed in a way that does not create a new substance

Physical changes and thermal energy

Thermal energy is heat energy. When matter gains or loses thermal energy, it can change from one state to another.

These state changes are physical changes because the substance stays the same.

• Melting: a solid changes to a liquid
• Freezing: a liquid changes to a solid
• Evaporation: a liquid changes to a gas
• Condensation: a gas changes to a liquid

Here is one example with water:

• Ice is solid water
• When ice gains thermal energy, it melts into liquid water
• When liquid water gains more thermal energy, it can evaporate into water vapor
• When water vapor loses thermal energy, it can condense back into liquid water
• When liquid water loses more thermal energy, it can freeze into ice

Even though water looks different in each state, it is still the same substance: water.

Other kinds of physical changes

Not all physical changes are about heating or cooling. Some happen when a force changes an object.

• Bending a spoon changes its shape
• Tearing cloth changes its size and shape
• Crushing a can changes its form
• Breaking chalk into pieces changes its size

In each case, the material is still the same material. No new substance is made.

Dissolving can be a physical change

When some substances dissolve in water, it may look like they disappear. But they are still there.

For example, when sugar dissolves in water, the sugar spreads out into tiny pieces that are too small to see easily. The sugar is still sugar. This means dissolving sugar in water is a physical change.

If the water evaporates, the sugar can be left behind again. This helps show that no new substance was made.

How to tell if a change is physical

Ask yourself these questions:

1. Did the substance only change in state, shape, size, or appearance?
2. Is it still the same kind of matter as before?
3. Was no new substance made?

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

Clues that a change is physical

• The material can be cut, torn, bent, melted, frozen, or crushed
• The substance before and after the change is the same
• The change may sometimes be reversed, like freezing water after it melts

Be careful: not every change in appearance means a new substance

Sometimes a substance looks very different after a physical change. For example, shaved ice and an ice cube do not look the same, but both are still water.

A paper towel ripped into pieces still has the same material as before. The appearance changed, but the substance did not.

Worked Example 1: Ice melting

Question: An ice cube sits on a plate and turns into a puddle of water. Is this a physical change?

Step 1: Identify what changed. The ice changed from a solid to a liquid.

Step 2: Ask if it is still the same substance. Yes, both ice and liquid water are water.

Answer: Yes, this is a physical change because only the state changed.

Worked Example 2: Cutting an apple

Question: A whole apple is sliced into pieces. Is this a physical change?

Step 1: Identify what changed. The apple changed in size and shape.

Step 2: Ask if it is still the same substance. Yes, it is still apple.

Answer: Yes, cutting an apple is a physical change.

Worked Example 3: Steam on a bathroom mirror

Question: Warm water in a shower makes water vapor. Then drops of water appear on the mirror. Is this a physical change?

Step 1: Water changed from liquid to gas in the shower. That is evaporation.

Step 2: The water vapor touched the cooler mirror and changed from gas to liquid. That is condensation.

Step 3: Ask if the substance stayed the same. Yes, it stayed water the whole time.

Answer: Yes, both evaporation and condensation are physical changes.

Worked Example 4: Sugar dissolving in water

Question: A spoonful of sugar is stirred into warm water until you cannot see it anymore. Is this a physical change?

Step 1: The sugar looks like it disappeared, but it has spread through the water.

Step 2: Ask if the sugar is still sugar. Yes, it is.

Step 3: If the water dries up, the sugar can be found again.

Answer: Yes, dissolving sugar in water is a physical change.

Try thinking about these examples

• Melting butter
• Freezing juice into a popsicle
• Breaking a crayon
• Flattening clay
• Boiling water into steam

These are all physical changes because the substance stays the same, even though its form changes.

Important idea to remember

In a physical change, the matter is still made of the same substance before and after the change. The change may affect how it looks, feels, or what state it is in, but it does not turn into something new.

Summary

A physical change changes the state, shape, size, or appearance of matter without changing what the substance is. Thermal energy can cause physical changes such as melting, freezing, evaporation, and condensation. Cutting, crushing, bending, and dissolving can also be physical changes when no new substance forms.

Phase Transitions: Heating means learning what happens when matter gains thermal energy, or heat. When something is heated, its tiny particles begin to move faster. As they move faster, matter can change from one state to another.

The three most common states of matter are solid, liquid, and gas. During heating, matter can change from solid to liquid, liquid to gas, or sometimes solid straight to gas.

These changes are called phase transitions. In this lesson, we will focus on four important heating changes: melting, boiling, evaporation, and sublimation.

Important idea: During a phase change, the matter is still the same substance. Only its state changes. For example, ice, liquid water, and water vapor are all made of water.

1. Matter is made of tiny particles

All matter is made of tiny particles too small to see. These particles are always moving.

• In a solid, particles are packed closely together and mainly vibrate in place.
• In a liquid, particles are still close together, but they can slide past one another.
• In a gas, particles are spread far apart and move freely and quickly.

When heat is added, particles absorb thermal energy. This gives them more energy to move.

2. Heating causes particles to move faster

Thermal energy transfer happens when heat moves from something warmer to something cooler. If a cooler material is heated, it absorbs that energy.

As particles absorb energy, they move faster. If they gain enough energy, they can break away from their current arrangement and change state.

This is why heating can cause phase transitions.

3. Melting: solid to liquid

Melting happens when a solid absorbs enough heat that its particles can no longer stay locked in place. The particles begin to move around each other, and the solid becomes a liquid.

A common example is ice melting into water. At first, the particles in ice only vibrate. As the ice absorbs heat, the particles move more and more until they can slide past each other as liquid water.

During melting:

• The substance absorbs heat.
• The particles move faster.
• The particles loosen from their fixed positions.
• The state changes from solid to liquid.

4. Boiling: liquid to gas throughout the liquid

Boiling happens when a liquid is heated enough that particles throughout the liquid change into gas. You can see this as bubbles forming inside the liquid.

When water boils, its particles absorb so much energy that they move fast enough to spread far apart and become water vapor, a gas.

During boiling:

• The liquid absorbs heat.
• Particles move much faster.
• Bubbles of gas form throughout the liquid.
• The state changes from liquid to gas.

5. Evaporation: liquid to gas at the surface

Evaporation is also a change from liquid to gas, but it is different from boiling. Evaporation happens only at the surface of a liquid.

Even when water is not boiling, some particles at the surface can gain enough energy to escape into the air as gas. That is why puddles dry up and wet clothes become dry.

During evaporation:

• The liquid absorbs heat from its surroundings.
• Only some particles at the surface escape.
• It can happen slowly.
• It does not need bubbling like boiling.

Boiling and evaporation are alike because both change liquid into gas. They are different because boiling happens throughout the liquid, while evaporation happens only at the surface.

6. Sublimation: solid to gas

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

One example is dry ice. Dry ice does not melt into a liquid puddle. Instead, it changes straight from solid dry ice into carbon dioxide gas.

Snow and ice can also slowly sublime in very cold, dry air. That means some solid water changes straight into water vapor.

During sublimation:

• The solid absorbs heat.
• Particles gain enough energy to break away completely.
• The state changes from solid to gas.
• There is no liquid stage in between.

7. What happens to the particles during each heating change?

• Melting: particles go from vibrating in place to sliding past each other.
• Boiling: particles in a liquid gain enough energy to spread far apart as gas.
• Evaporation: some surface particles in a liquid escape into the air as gas.
• Sublimation: particles in a solid gain enough energy to go straight into gas.

In every case, the particles absorb energy. That absorbed energy is what makes the phase transition happen.

8. The substance stays the same during a phase change

It is important to remember that phase changes are physical changes, not chemical changes. A physical change changes the form or state of matter, but not what it is made of.

For example:

• Ice is water in the solid state.
• Liquid water is water in the liquid state.
• Water vapor is water in the gas state.

All three are still water.

This is different from a chemical change, where a new substance is formed. In this lesson, melting, boiling, evaporation, and sublimation do not make new substances.

9. Heating examples in everyday life

• An ice cube on a table melts into liquid water.
• Water in a pot on the stove boils into steam.
• A rain puddle on a sunny day evaporates and disappears.
• Dry ice in a science demonstration sublimes into gas.

Each example shows matter absorbing thermal energy and changing state.

Worked Example 1: Melting

Question: A student leaves a butter stick on a warm kitchen counter. After a while, it becomes soft and starts turning into liquid. What phase transition is happening, and what are the particles doing?

Step 1: Identify the starting state. Butter starts as a solid.

Step 2: Identify the ending state. It becomes liquid.

Step 3: Name the phase transition. Solid to liquid is melting.

Step 4: Describe particle behavior. The particles absorb heat, move faster, and are no longer locked in place. They begin to slide past one another.

Answer: The butter is melting. Its particles are absorbing energy and moving more freely.

Worked Example 2: Boiling or Evaporation?

Question: A pot of water is on the stove. Bubbles are rising all through the water. Is this boiling or evaporation?

Step 1: Look for clues. The question says bubbles are rising through the water.

Step 2: Match the clue to the process. Bubbles throughout the liquid mean boiling.

Step 3: Explain particle behavior. The water particles absorbed enough heat to change into gas throughout the liquid.

Answer: This is boiling, because gas is forming throughout the liquid.

Worked Example 3: Drying a Shirt

Question: A wet shirt is hung outside. Later, it is dry. Which phase transition happened to the water on the shirt?

Step 1: Identify the starting state. The water on the shirt is a liquid.

Step 2: Identify the ending state. The water goes into the air as a gas.

Step 3: Decide whether it is boiling or evaporation. The shirt is not bubbling, and the water leaves from the surface slowly.

Answer: The water changed by evaporation. Surface particles absorbed energy and escaped into the air as gas.

Worked Example 4: Solid to Gas

Question: In a science lab, a piece of dry ice gets smaller and gives off a foggy-looking cloud. It does not turn into a liquid first. What phase transition is this?

Step 1: Identify the starting state. Dry ice begins as a solid.

Step 2: Identify the ending state. It becomes a gas.

Step 3: Check whether there is a liquid stage. The question says there is no liquid first.

Answer: This is sublimation, a change directly from solid to gas.

10. Quick compare chart

• Melting: solid \(\rightarrow\) liquid
• Boiling: liquid \(\rightarrow\) gas throughout the liquid
• Evaporation: liquid \(\rightarrow\) gas at the surface
• Sublimation: solid \(\rightarrow\) gas

You can think of heating changes like this:

More absorbed thermal energy \(\rightarrow\) faster particle motion \(\rightarrow\) possible change of state

We can write that as:

$$\text{heat absorbed} \rightarrow \text{particles move faster} \rightarrow \text{phase change}$$

11. Common mistakes to avoid

• Mistake: Thinking boiling and evaporation are the same exact process.
  Remember: Both make gas, but boiling happens throughout the liquid and evaporation happens only at the surface.
• Mistake: Thinking a new substance forms during melting or boiling.
  Remember: Phase changes are physical changes. The substance stays the same.
• Mistake: Thinking particles get bigger when heated.
  Remember: The particles do not get bigger. They move faster and farther apart.
• Mistake: Forgetting that sublimation skips the liquid state.
  Remember: Sublimation is solid straight to gas.

12. Brief Summary

Heating adds thermal energy to matter. As matter absorbs this energy, its particles move faster.

If the particles gain enough energy, matter can change state. Melting changes a solid to a liquid, boiling changes a liquid to a gas throughout the liquid, evaporation changes a liquid to a gas at the surface, and sublimation changes a solid directly to a gas.

In all of these phase transitions, the substance stays the same. Only its state changes.

Phase Transitions: Cooling means matter changes from one state to another when it loses thermal energy. Thermal energy is the energy that makes particles move. When matter cools, its particles slow down, move less, and often get closer together.

In this lesson, you will learn how cooling causes three important phase changes: condensation, freezing, and deposition. You will also trace what the tiny particles are doing and understand that energy is released to the surroundings during these changes.

Remember: Matter is made of tiny particles. We cannot usually see them, but we can use a particle model to imagine how they move.

The three common states of matter are:

• Solid: particles are packed close together and mostly vibrate in place.
• Liquid: particles are close together but can slide past each other.
• Gas: particles are spread far apart and move quickly in all directions.

When matter cools, particles lose thermal energy. As a result:

• they move more slowly,
• they may come closer together, and
• the matter can change state.

Cooling phase changes happen because energy moves out of the matter and into the surroundings. For example, warm water can lose energy to cold air. This is called energy release.

Here is a simple cooling path:

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

Now let’s look closely at each cooling phase transition.

1. Condensation

Condensation is the change from a gas to a liquid.

During condensation, gas particles lose thermal energy. They slow down and move closer together. When this happens, the gas turns into liquid droplets.

A common example is water vapor in the air turning into liquid water on the outside of a cold glass. The air near the glass cools down. The water vapor loses energy and becomes tiny drops of liquid water.

Particle view of condensation:

• Before: particles are far apart and moving fast.
• During cooling: particles lose energy and slow down.
• After: particles are closer together and can slide past each other as a liquid.

2. Freezing

Freezing is the change from a liquid to a solid.

During freezing, liquid particles lose more thermal energy. They slow down so much that they can no longer slide around easily. Instead, they stay in fixed positions and mostly vibrate. The liquid becomes a solid.

A common example is liquid water turning into ice in a freezer. The water gives off thermal energy to the colder air in the freezer. As the water particles slow down, they form solid ice.

Particle view of freezing:

• Before: particles are close together and sliding past one another.
• During cooling: particles lose energy and slow down more.
• After: particles are tightly arranged and mostly vibrate in place.

3. Deposition

Deposition is the change from a gas directly to a solid.

This phase change skips the liquid state. During deposition, gas particles lose a lot of thermal energy very quickly. They slow down, move close together, and form a solid.

A good example is frost forming on a cold window or on grass. Water vapor in the air loses energy to the cold surface and turns directly into solid ice crystals.

Particle view of deposition:

• Before: particles are far apart and moving fast as a gas.
• During cooling: particles lose a lot of energy.
• After: particles become packed in a solid and mostly vibrate in place.

How is cooling different in each phase change?

• Condensation: gas loses energy and becomes liquid.
• Freezing: liquid loses energy and becomes solid.
• Deposition: gas loses energy and becomes solid.

All three changes happen because thermal energy leaves the matter. The surroundings take in that energy.

Important idea: particles do not disappear. The same matter is still there. Only the way the particles move and arrange themselves changes.

For example, water can be:

• water vapor as a gas,
• liquid water as a liquid, or
• ice as a solid.

It is still water in all three states. Cooling changes its state, not what kind of matter it is.

Temperature and phase changes

Temperature tells us how warm or cool something is. In many cases, as matter cools, its temperature goes down. When enough energy is removed, a phase change can happen.

For water, freezing happens at about \(0^\circ\text{C}\), which is \(32^\circ\text{F}\).

We can write that as:

$$0^\circ\text{C} = 32^\circ\text{F}$$

This means liquid water can freeze into ice when it cools to that temperature or below under normal conditions.

Signs that cooling phase changes are happening

• droplets form on a cold surface,
• a liquid becomes hard,
• frost appears on a very cold surface,
• particles are slowing down and moving less.

Worked Example 1: Condensation on a cup

A glass of ice water is placed on a table. Soon, small drops of water appear on the outside of the glass. Where did the water come from?

Step 1: The outside of the glass is very cold.

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

Step 3: The water vapor loses thermal energy.

Step 4: The gas changes to liquid water.

Answer: The drops came from water vapor in the air. This is condensation.

Worked Example 2: Freezing water into ice

A tray filled with liquid water is placed in a freezer. After some time, the water becomes ice.

What are the particles doing?

1. At first, the liquid water particles are close together and moving past each other.
2. The freezer removes thermal energy from the water.
3. The particles slow down.
4. The particles settle into fixed positions and mostly vibrate.

Answer: The water went through freezing, changing from a liquid to a solid because it lost thermal energy.

Worked Example 3: Frost on grass

Early in the morning, the grass has frost on it, even though there was no rain. How did the frost form?

Step 1: Water vapor was in the air.

Step 2: The grass was very cold.

Step 3: The water vapor lost thermal energy when it touched the cold grass.

Step 4: The gas changed directly into solid ice crystals.

Answer: The frost formed by deposition, which is a change from gas directly to solid.

Worked Example 4: Choosing the correct phase change

Look at each situation and name the cooling phase change.

• Cloud water droplets form from water vapor.
• Juice becomes a popsicle.
• Frost forms inside a freezer.

Solution:

• Water vapor to liquid droplets = condensation
• Liquid juice to solid popsicle = freezing
• Water vapor to solid frost = deposition

A helpful way to remember

• Condensation: gas to liquid
• Freezing: liquid to solid
• Deposition: gas to solid

You can also think about particle motion:

• more energy = faster particle motion,
• less energy = slower particle motion.

So when matter cools, the particles slow down. That slowing down helps matter change into a liquid or a solid.

Common mistakes to avoid

• Do not say the water on the outside of a cold glass leaked through the glass. It usually came from the air by condensation.
• Do not mix up freezing and deposition. Freezing starts with a liquid, but deposition starts with a gas.
• Do not forget that cooling means thermal energy leaves the matter.

Brief Summary

Cooling can cause matter to change state because particles lose thermal energy, slow down, and move closer together. Condensation is gas to liquid, freezing is liquid to solid, and deposition is gas directly to solid. In every cooling phase change, energy is released from the matter to the surroundings.

Phase Changes happen when matter changes from one state to another.

Matter is everything that takes up space. Matter can be a solid, a liquid, or a gas.

• Solid: keeps its shape, like an ice cube.
• Liquid: flows and takes the shape of its cup, like water.
• Gas: spreads out in the air, like water vapor.

When we add heat or take heat away, matter can change state. This is called a phase change.

Think of heat like energy that can help matter change. Adding heat can make matter move into a new state. Removing heat can make it change back.

Main Phase Changes

• Melting: a solid changes to a liquid.
• Freezing: a liquid changes to a solid.
• Vaporization: a liquid changes to a gas.
• Condensation: a gas changes to a liquid.
• Sublimation: a solid changes right into a gas.

1. Melting

Melting happens when a solid gets warmer and turns into a liquid.

An ice cube on a table melts into water. The ice gets heat from the air around it.

So:

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

2. Freezing

Freezing happens when a liquid gets colder and turns into a solid.

Water in an ice tray freezes in the freezer and becomes ice.

So:

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

3. Vaporization

Vaporization happens when a liquid gets warmer and turns into a gas.

When water is heated, some of it can become water vapor and go into the air.

You may see this when a pot of water gets very hot.

So:

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

4. Condensation

Condensation happens when a gas gets cooler and turns into a liquid.

Water vapor in the air can cool down and make tiny drops of water on the outside of a cold cup.

So:

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

5. Sublimation

Sublimation is a special phase change. It happens when a solid changes right into a gas.

This means it skips the liquid state.

Dry ice can do this. It changes from a solid to a gas.

So:

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

Adding Heat and Removing Heat

Here is a simple way to remember the changes:

• Adding heat: melting, vaporization, sublimation
• Removing heat: freezing, condensation

We can show it like this:

$$\text{solid} \xrightarrow{\text{add heat}} \text{liquid} \xrightarrow{\text{add heat}} \text{gas}$$

$$\text{gas} \xrightarrow{\text{remove heat}} \text{liquid} \xrightarrow{\text{remove heat}} \text{solid}$$

Why does this happen?

When matter gets more heat, its tiny pieces move more. This can help a solid melt or a liquid turn into gas.

When matter loses heat, its tiny pieces slow down. This can help a gas turn into liquid or a liquid freeze into a solid.

You do not need to see the tiny pieces to know they are there. We can tell by watching the phase change happen.

Examples in Real Life

• An ice pop left outside melts.
• Water in a freezer freezes into ice cubes.
• A puddle dries up because liquid water changes into gas and goes into the air.
• Drops of water form on a bathroom mirror after a hot shower.
• Dry ice changes from solid to gas.

Worked Example 1

You put an ice cube in a bowl and leave it on the counter.

What happens?

The ice cube gets heat from the room. It changes from a solid to a liquid.

Answer: It melts.

Worked Example 2

You pour water into an ice tray and put it in the freezer.

What happens?

The water loses heat. It changes from a liquid to a solid.

Answer: It freezes.

Worked Example 3

You see drops of water on the outside of a very cold glass.

What happened?

Water vapor in the air touched the cold glass. The gas cooled and changed into liquid drops.

Answer: That is condensation.

Worked Example 4

A puddle is gone after a sunny day.

What happened?

The liquid water got heat from the Sun and changed into a gas that went into the air.

Answer: That is vaporization.

How to Tell Which Phase Change It Is

1. Ask: What state did it start as?
2. Ask: What state did it end as?
3. Ask: Did it gain heat or lose heat?

For example, if something starts as liquid water and ends as ice, it changed from liquid to solid, so it is freezing.

Quick Review

• Melting: solid to liquid
• Freezing: liquid to solid
• Vaporization: liquid to gas
• Condensation: gas to liquid
• Sublimation: solid to gas

Summary

Phase changes are ways matter changes from one state to another.

When heat is added, matter can melt, vaporize, or sublimate. When heat is removed, matter can condense or freeze.

If you remember the starting state and the ending state, you can name the phase change.

Thermal Expansion and Contraction

Have you ever noticed that some things seem to change size when they get hot or cold? This happens because of thermal expansion and contraction.

Thermal expansion means matter gets a little bigger when it is heated. Thermal contraction means matter gets a little smaller when it is cooled.

This happens because all matter is made of tiny particles. When matter gains thermal energy, its particles move faster and spread a little farther apart. When matter loses thermal energy, its particles move slower and move a little closer together.

As the distance between particles changes, the volume of an object can change. Volume is the amount of space something takes up.

Big idea: Heating usually causes expansion, and cooling usually causes contraction.

How particles behave

Even though we cannot see particles with our eyes, we can use a model to understand what happens.

• When matter is heated: particles gain thermal energy
• They move faster
• They spread farther apart
• The object expands and takes up more space

• When matter is cooled: particles lose thermal energy
• They move slower
• They move closer together
• The object contracts and takes up less space

This idea works for solids, liquids, and gases, but the amount of change is not always the same.

Thermal expansion in solids

In solids, particles are packed closely and stay in fixed positions, but they still move by vibrating. When a solid is heated, the particles vibrate faster and push a little farther apart. This makes the solid expand slightly.

When the solid cools, the particles vibrate less and move a bit closer together. The solid contracts slightly.

Solids do expand and contract, but the change is often small. Even small changes can matter in buildings, bridges, and roads.

Example: Gaps are left between sections of a bridge so the metal can expand on hot days without bending or breaking.

Thermal expansion in liquids

In liquids, particles can move past one another. When a liquid is heated, its particles move faster and spread out more. The liquid takes up more space.

This is why the liquid in a thermometer rises when it gets warmer. The liquid expands and moves upward in the narrow tube.

When the liquid cools, it contracts and takes up less space, so the level falls.

Thermal expansion in gases

Gas particles are already far apart and move freely. When a gas is heated, its particles move much faster and spread out even more. Gases usually expand more than solids and liquids do.

When a gas cools, its particles slow down and come closer together. The gas contracts.

Example: A balloon left in a warm place may get bigger because the gas inside expands. A balloon in a cold place may shrink because the gas inside contracts.

Why volume changes

Remember, thermal expansion and contraction do not usually change the amount of matter in an object. The object still has the same matter in it. What changes is the space between the particles.

If the particles spread out, the object takes up more space. If the particles move closer together, the object takes up less space.

We can describe the idea like this:

When heated:

$$\text{particles farther apart} \rightarrow \text{more volume}$$

When cooled:

$$\text{particles closer together} \rightarrow \text{less volume}$$

Important note: The change in size is often small, but it is real and useful to understand.

Everyday examples

• Roads and sidewalks: They can crack if materials expand in hot weather and do not have room to move.
• Bridges: Engineers leave spaces called expansion gaps so the bridge can safely expand and contract.
• Jar lids: Running warm water over a metal lid can help loosen it because the metal expands.
• Thermometers: The liquid inside expands when heated and contracts when cooled.
• Power lines: On hot days they may sag more because the metal expands. On cold days they tighten because the metal contracts.

Worked Example 1: A metal spoon in hot soup

Question: A metal spoon sits in a bowl of hot soup. What happens to the spoon as it gets warmer?

Step 1: The hot soup transfers thermal energy to the spoon.

Step 2: The spoon's particles gain thermal energy and vibrate faster.

Step 3: The particles move slightly farther apart.

Answer: The spoon expands slightly. It becomes a tiny bit larger, even though the change may be too small to notice easily.

Worked Example 2: A balloon in the refrigerator

Question: A balloon is placed in a refrigerator. What happens to the gas inside and to the balloon?

Step 1: The gas inside the balloon loses thermal energy.

Step 2: The gas particles move slower.

Step 3: The particles move a little closer together.

Answer: The gas contracts, so the balloon gets smaller.

Worked Example 3: Bridge gaps

Question: Why do builders leave small gaps between parts of a bridge?

Step 1: Bridges can heat up during the day and in summer.

Step 2: The metal or concrete particles gain thermal energy and move farther apart.

Step 3: The bridge materials expand.

Answer: Builders leave gaps so the bridge has room to expand. Without gaps, the bridge could bend, crack, or break.

Worked Example 4: Comparing hot and cold water in a bottle

Question: Two identical bottles are filled to the same level, one with hot water and one with cold water. Which water takes up more space?

Step 1: Water is a liquid, so its particles can move past one another.

Step 2: In the hot bottle, the particles have more thermal energy and spread out more.

Step 3: In the cold bottle, the particles have less thermal energy and stay closer together.

Answer: The hot water takes up more space because it has expanded.

How this connects to changes in matter

Thermal expansion and contraction are changes caused by thermal energy transfer. Matter may stay the same kind of substance, but its volume changes because the particles change how far apart they are.

This is different from a chemical change. In thermal expansion and contraction, no new substance is made. The matter is still the same material. It is only changing in size because of heating or cooling.

For example, heating a metal rod makes it expand, but it is still metal. Cooling the rod makes it contract, but it is still the same metal.

Quick check for understanding

1. What usually happens to matter when it is heated?
2. What happens to particles when matter cools?
3. Why does a thermometer's liquid rise when it gets warmer?
4. Why are expansion gaps important in bridges and roads?

Answers:

1. It usually expands.
2. The particles move slower and get closer together.
3. The liquid expands and takes up more space.
4. They give materials room to expand safely when heated.

Summary

Thermal expansion and contraction happen because heating and cooling change how particles move. When matter gains thermal energy, particles move faster and spread out, so the matter expands. When matter loses thermal energy, particles move slower and come closer together, so the matter contracts.

This can happen in solids, liquids, and gases. Understanding this idea helps explain many things we see in everyday life, from thermometers and balloons to bridges and roads.

Solubility Dynamics is the study of what makes something dissolve faster or slower. In this lesson, we will learn how temperature, surface area, and agitation change the rate of dissolving.

Have you ever stirred sugar into a drink? At first, you can see the sugar. After a while, it seems to disappear. It did not vanish. The sugar spread out into the liquid so well that you can no longer see the pieces. This is called dissolving.

When a substance dissolves, the substance being dissolved is called the solute. The liquid doing the dissolving is called the solvent. For example, in sugar water, sugar is the solute and water is the solvent.

The word solubility tells us how well a solute can dissolve in a solvent. In this lesson, we are mostly focusing on how fast dissolving happens, not just whether it can happen.

Think of dissolving as tiny particles mixing together. The particles in the solvent bump into the particles in the solute. These bumps help pull the solute apart and spread it through the liquid.

Three important things can change how quickly a solute dissolves:

• Temperature — how hot or cold the solvent is
• Surface area — how much of the solute is exposed to the solvent
• Agitation — how much the mixture is stirred or shaken

1. Temperature

Temperature is a measure of how hot or cold something is. When water is warmer, its particles move faster. Faster-moving water particles bump into the solute more often.

Because of this, many solids, like sugar or salt, dissolve faster in warm water than in cold water. This is why hot tea can dissolve sugar quickly.

Imagine two cups of water. One is cold, and one is warm. If you put the same amount of sugar into each cup, the sugar will usually dissolve faster in the warm cup.

This does not always mean more will dissolve forever, but it often means the dissolving happens more quickly.

2. Surface Area

Surface area means how much of the outside of an object is showing. Smaller pieces have more surface area than one big piece of the same total amount.

For example, crushed sugar dissolves faster than a sugar cube. Why? More tiny pieces are touching the water at the same time.

If a solute has more surface area, the solvent can bump into more of it at once. That helps the solute dissolve faster.

You can think of it like this: one big rock has less outside space touching water than many tiny pebbles made from the same amount of rock.

3. Agitation

Agitation means mixing by stirring, shaking, or swirling. When you stir a drink, fresh water moves past the solute again and again.

This helps carry dissolved particles away and brings new solvent particles into contact with the solute. That makes dissolving happen faster.

That is why sugar at the bottom of a cup dissolves more quickly when you stir the cup than when you leave it still.

How These Factors Work Together

Temperature, surface area, and agitation can work together. If you use warm water, crush the solute into smaller pieces, and stir, the solute will often dissolve very quickly.

If you use cold water, leave the solute in a big chunk, and do not stir, dissolving will usually be much slower.

Important Idea: Faster dissolving does not mean the solute disappeared. The particles are still there, mixed into the solvent.

Worked Example 1: Stirring vs. Not Stirring

A student puts one spoonful of sugar into two cups of room-temperature water. She stirs one cup and leaves the other cup alone.

Question: Which cup will dissolve the sugar faster?

Answer: The stirred cup will dissolve the sugar faster.

Why: Stirring is a form of agitation. It keeps moving water around the sugar, so the sugar mixes into the water more quickly.

Worked Example 2: Warm Water vs. Cold Water

Two equal sugar cubes are placed into two cups. One cup has warm water. The other has cold water. Neither cup is stirred.

Question: In which cup will the sugar cube dissolve faster?

Answer: The sugar cube will dissolve faster in the warm water.

Why: Warm water particles move faster and bump into the sugar more often.

Worked Example 3: Sugar Cube vs. Crushed Sugar

A student places one sugar cube in a glass of water. In another glass, the student places the same amount of sugar, but crushed into tiny pieces. Both glasses have the same temperature water and are stirred the same amount.

Question: Which glass will have the sugar dissolve faster?

Answer: The crushed sugar will dissolve faster.

Why: The crushed sugar has more surface area. More of the sugar is touching the water at one time.

Worked Example 4: Putting It All Together

Four students are trying to dissolve the same amount of salt in water.

• Student A uses cold water and does not stir.
• Student B uses warm water and does not stir.
• Student C uses cold water and stirs.
• Student D uses warm water, crushes the salt into finer pieces, and stirs.

Question: Whose salt will likely dissolve the fastest?

Answer: Student D's salt will likely dissolve the fastest.

Why: Student D uses all three factors that speed up dissolving: warm temperature, greater surface area, and agitation.

A Simple Way to Compare Dissolving Rate

If one sample dissolves in 2 minutes and another dissolves in 6 minutes, the one that takes 2 minutes is dissolving faster.

We can compare times like this:

$$2 \text{ minutes} < 6 \text{ minutes}$$

So the sample that dissolved in 2 minutes had the faster dissolving rate.

Here is another way to think about it:

$$\text{shorter time} = \text{faster dissolving}$$

Real-Life Examples

• Sugar dissolves faster in hot cocoa than in iced tea.
• Powdered drink mix dissolves faster than a large hard chunk.
• Stirring soup helps salt spread through it more quickly.
• Shaking a bottle helps some powders mix into liquid faster.

Things to Remember

• A solute is what gets dissolved.
• A solvent is what does the dissolving.
• Temperature can make dissolving happen faster.
• More surface area can make dissolving happen faster.
• Agitation can make dissolving happen faster.
• Dissolving faster does not mean the matter is gone.

Quick Check for Understanding

1. If you want sugar to dissolve faster, should you use warm or cold water?
2. Why does crushed candy dissolve faster than one whole piece?
3. How does stirring help a solute dissolve?

Brief Summary

Solubility dynamics helps us understand what changes the rate of dissolving. Warm liquids, smaller pieces, and stirring usually help a solute dissolve faster. These ideas explain many everyday things, like why sugar dissolves quickly in hot drinks and why stirring helps mix ingredients.

Indicators of Chemical Change

Sometimes matter changes in a simple way, and sometimes it changes into a new substance. In science, a chemical change happens when materials react and form something new.

We can often tell a chemical change is happening by looking for indicators, or clues. These clues do not prove every reaction by themselves, but they are strong signs that a new substance may have formed.

In this lesson, you will learn the main indicators of chemical change:

• Unexpected color change
• Temperature change
• Gas production
• Precipitate formation

You will also learn how chemical changes are different from physical changes. A physical change only changes the form of matter, like size, shape, or state. For example, ice melting into water is a physical change because it is still water.

But when a chemical change happens, the starting materials and the ending materials are not the same. A new substance has formed.

1. Unexpected Color Change

One clue of a chemical change is an unexpected color shift. This means the material changes color in a way that shows something new may have formed.

For example, when iron rusts, it changes from gray to a reddish-brown color. That color change is a sign that the iron has reacted with oxygen and formed rust.

Not every color change means a chemical change. If you mix blue paint and yellow paint to make green, that is just mixing colors. No new substance is formed. So scientists look for color change along with other clues.

2. Temperature Change

A chemical reaction can cause the temperature to change. Sometimes the materials get warmer. Sometimes they get cooler.

This happens because energy is transferred during the reaction. You may notice this without heating or cooling the materials from the outside.

For example, some reactions give off heat. If two substances are mixed and the container feels warm, that can be a sign of chemical change. Other reactions take in energy and feel cooler.

A temperature change by itself is not always a chemical change. Heating water until it boils is a physical change because the water is still water. The key question is: Did a new substance form?

3. Gas Production

Another indicator is gas production. When a chemical reaction makes a gas, you may see bubbles, foam, or fizzing.

For example, when vinegar and baking soda are mixed, they fizz and bubble. That bubbling is a sign that a gas is being produced during the reaction.

Be careful: bubbles do not always mean a chemical change. If water boils, it makes bubbles of water vapor. That is a physical change, not a chemical one. So again, we ask whether a new substance was made.

4. Precipitate Formation

A precipitate is a new solid that forms when two liquids are mixed together.

At first, both materials may look like clear liquids. But after mixing, a cloudy solid may appear. That solid is called a precipitate, and it is an important sign of chemical change.

For example, if two clear solutions are combined and suddenly a white solid forms, that shows a new substance has been made.

How Is a Chemical Change Different from a Physical Change?

It is important to compare the two kinds of changes.

• Physical change: changes size, shape, or state, but does not make a new substance.
• Chemical change: forms one or more new substances.

Here are some examples of physical changes:

• Ice melting
• Water freezing
• Tearing paper
• Crushing a can

Here are some examples of chemical changes:

• Wood burning
• Iron rusting
• Baking a cake
• Vinegar reacting with baking soda

Why Do Scientists Look for More Than One Clue?

Sometimes one sign can be confusing. A color change or bubbles alone may happen during a physical change. That is why scientists often look for several indicators together.

If you see an unexpected color change, gas production, and a temperature change, it is much more likely that a chemical reaction has happened.

Worked Example 1: Rust on a Bike Chain

Situation: A shiny gray bike chain is left outside in the rain. After some time, parts of it become reddish-brown.

Think: Did the chain just get wet, or did a new substance form?

Answer: This is a chemical change. The unexpected color change is a clue. The iron reacted with oxygen and water in the air, forming rust, which is a new substance.

Worked Example 2: Water Boiling in a Pot

Situation: Water is heated on a stove. Bubbles rise as the water boils.

Think: Are the bubbles a sign of chemical change?

Answer: No. This is a physical change. The bubbles are water vapor. The water is changing state from liquid to gas, but it is still the same substance: water.

Worked Example 3: Vinegar and Baking Soda

Situation: A student mixes vinegar and baking soda in a cup. The mixture fizzes, bubbles, and feels cooler.

Think: Which indicators of chemical change are present?

Answer: There are two indicators here:

• Gas production because the mixture bubbles and fizzes
• Temperature change because the cup feels cooler

These clues show that a chemical reaction is happening and new substances are being formed.

Worked Example 4: Two Clear Liquids Make a Solid

Situation: A student mixes two clear liquids. After stirring, a cloudy solid appears in the liquid.

Think: What indicator does this show?

Answer: This shows precipitate formation. The new solid is a precipitate, which is strong evidence of a chemical change.

Quick Check: Is It a Chemical Change?

1. A puddle dries up in the sun.
   This is a physical change. The liquid water changes to water vapor.
2. A marshmallow turns black when burned.
   This is a chemical change. The color changes, and new substances form.
3. A cold pack becomes colder after being squeezed.
   This may be a chemical change because there is a temperature change caused by a reaction inside the pack.
4. Two liquids are mixed, and bubbles form.
   This may be a chemical change because gas is produced.

Helpful Tips to Remember

• If a new substance forms, it is a chemical change.
• Look for clues you can observe with your senses.
• The main indicators are color change, temperature change, gas production, and precipitate formation.
• One clue is helpful, but more than one clue gives stronger evidence.
• Changes of state, like melting and boiling, are usually physical changes.

Lesson Summary

Chemical changes happen when substances react and form new substances. Scientists look for indicators, or clues, to tell when this happens.

The main indicators of chemical change are an unexpected color change, a temperature change, gas production, and precipitate formation. These clues help us decide whether a change is chemical or physical.

When you study matter, always ask: Did a new substance form? If the answer is yes, then a chemical change has happened.

Chemical Changes happen when a material changes into a new kind of material.

This is different from a change in size, shape, or state. If you tear paper, melt ice, or break a crayon, it is still the same kind of matter. But in a chemical change, a new substance is made.

We may not be able to see tiny pieces of matter, but we can see clues that a chemical change happened. These clues help us know that something new was formed.

Big idea: A chemical change makes something new with different properties.

Here are some clues of a chemical change:

• Color changes in a surprising way
• A new smell appears
• Bubbles or gas form
• Heat or light is given off
• A new solid forms from liquids

Sometimes one clue is enough to make us wonder. When we see more than one clue, it is even more likely that a chemical change happened.

Let’s compare physical changes and chemical changes.

• Physical change: The matter looks different, but it is still the same kind of matter.
• Chemical change: The matter changes into a new kind of matter.

Physical change examples:

• Ice melting into water
• Paper being cut
• Clay being shaped into a ball

Chemical change examples:

• Baking a cake
• Cooking an egg
• Wood burning
• Iron rusting

When a cake bakes, the wet batter does not stay batter. Heat helps it turn into a cake. The cake has new properties. It smells different, tastes different, and feels different.

When an egg cooks, the runny egg turns solid. It also changes color and texture. You cannot turn the cooked egg back into the same raw egg. That is a chemical change.

When wood burns, it makes ash, smoke, heat, and light. Those are strong clues that a new substance formed. Burning is a chemical change.

When metal rusts, it slowly changes when it mixes with air and water. Rust is a new substance. So rusting is a chemical change too.

Worked Example 1

Question: A child cuts a piece of paper into small squares. Is this a chemical change?

Step 1: Ask, “Was a new substance made?”

Step 2: The paper is still paper. It is only smaller pieces now.

Answer: No. This is a physical change, not a chemical change.

Worked Example 2

Question: A marshmallow is roasted and turns brown with a new smell. Is this a chemical change?

Step 1: Look for clues.

• The color changed.
• A new smell appeared.
• Heat was used.

Step 2: These clues show something new was made.

Answer: Yes. Roasting the marshmallow is a chemical change.

Worked Example 3

Question: Ice melts in the sun. Is this a chemical change?

Step 1: What was it before? Ice.

Step 2: What is it after? Water.

Step 3: Ice and water are the same substance in different forms.

Answer: No. Melting ice is a physical change.

Worked Example 4

Question: A nail is left outside and later has reddish-brown rust on it. Is this a chemical change?

Step 1: Look for signs of a new substance.

Step 2: Rust is not the same as shiny metal. It has different color and different properties.

Answer: Yes. Rusting is a chemical change.

Here is a simple way to think about it:

1. Look at the matter before the change.
2. Look at the matter after the change.
3. Ask, “Is it still the same kind of matter?”
4. If something new formed, it is a chemical change.

Important reminder: Not every color change means a chemical change. But when color change happens with other clues, like smell, bubbles, or heat, it can be a strong sign.

You can also remember this rule:

Chemical change = new substance

Physical change = same substance

Scientists study changes in matter to understand the world. Cooking, burning, and rusting are all everyday examples of chemical changes.

Summary

A chemical change happens when matter turns into a new substance. Clues include color change, bubbles, new smell, heat, light, or a new solid. If the matter is still the same kind, it is a physical change. If something new is made, it is a chemical change.

Reactants and Products are the starting point and ending point of a chemical reaction. A chemical reaction happens when substances change and form new substances.

The substances you start with are called reactants. The new substances that are made are called products.

You can think of it like this: reactants go in, products come out.

In a simple reaction sentence, we can show it like this:

$$\text{reactants} \rightarrow \text{products}$$

The arrow means changes into or forms.

For example:

$$\text{vinegar} + \text{baking soda} \rightarrow \text{gas} + \text{other new substances}$$

Here, vinegar and baking soda are the reactants. The gas and the other new substances are the products.

Why This Matters

When scientists study matter, they want to know what changed. Looking at reactants and products helps us answer important questions:

• What substances started the reaction?
• What new substances were formed?
• How can we tell a chemical reaction happened?

Knowing the reactants and products helps us map the progression of a reaction from start to finish.

How to Identify Reactants and Products

To find the reactants and products, follow these steps:

1. Look for the substances at the beginning. These are the reactants.
2. Look for the substances made at the end. These are the products.
3. Find the arrow. Things before the arrow are reactants. Things after the arrow are products.

Here is the pattern again:

$$A + B \rightarrow C + D$$

• A and B are reactants.
• C and D are products.

Chemical Reactions Make New Substances

In a chemical reaction, the reactants do not just mix and stay the same. They change to make something new.

This is different from a physical change, like melting ice. When ice melts, it is still water. No new substance is made.

In a chemical reaction, the products are different substances from the reactants.

Some clues that a chemical reaction may have happened are:

• bubbles or gas forming
• a color change
• a new smell
• heat or light being given off
• a solid forming from liquids

These clues can help you notice that reactants have turned into products.

Reactants, Products, and Energy

Some chemical reactions need energy to get started. Thermal energy, or heat, can help reactants begin changing.

Other reactions give off energy as they happen. You may notice warmth, light, or both.

Even when energy is involved, the key idea stays the same: reactants are the starting substances, and products are the new substances formed.

Worked Example 1: Toasting Bread

When bread is heated in a toaster for long enough, it changes color, smells different, and tastes different. New substances form during toasting.

Question: What are the reactants and products?

Step 1: Identify the starting substance.

• Bread is the starting substance, so it is the reactant.

Step 2: Identify the new substance formed.

• Toasted bread is the product.

Answer: Bread is the reactant, and toasted bread is the product.

We can write it like this:

$$\text{bread} \rightarrow \text{toast}$$

Worked Example 2: Vinegar and Baking Soda

When vinegar and baking soda are mixed, they fizz and make bubbles. The bubbles show that a gas is being formed.

Question: What are the reactants and products?

Step 1: Look at what you start with.

• Vinegar
• Baking soda

These are the reactants.

Step 2: Look at what is made.

• A gas
• Other new substances in the mixture

These are the products.

Answer: Vinegar and baking soda are reactants. The gas and other new substances are products.

We can write:

$$\text{vinegar} + \text{baking soda} \rightarrow \text{gas} + \text{new substances}$$

Worked Example 3: Iron Rusting

When iron is left outside for a long time, it can react with oxygen in the air and form rust.

Question: What are the reactants and product?

Step 1: Find the starting substances.

• Iron
• Oxygen

These are the reactants.

Step 2: Find the new substance formed.

• Rust

This is the product.

Answer: Iron and oxygen are reactants. Rust is the product.

We can write:

$$\text{iron} + \text{oxygen} \rightarrow \text{rust}$$

Worked Example 4: Cooking an Egg

When an egg is heated, it changes color and texture. The cooked egg is different from the raw egg.

Question: What is the reactant, and what is the product?

Step 1: Start with the beginning substance.

• Raw egg = reactant

Step 2: Find the new substance after heating.

• Cooked egg = product

Answer: The raw egg is the reactant, and the cooked egg is the product.

We can write:

$$\text{raw egg} \rightarrow \text{cooked egg}$$

How to Map a Reaction from Start to Finish

To map the progression of a chemical reaction, think about the reaction in order.

1. Start: What substances are present first? These are the reactants.
2. Interaction: The reactants are mixed, heated, or changed in some way.
3. Evidence: Look for signs of a chemical reaction, such as bubbles, color change, or heat.
4. Finish: Identify the new substances formed. These are the products.

Here is an example with words:

Start: baking soda and vinegar

Interaction: they are mixed together

Evidence: fizzing and bubbles appear

Finish: gas and other new substances are formed

This helps you tell the story of the reaction from beginning to end.

Important Idea: Not Every Change Is a Chemical Reaction

It is important not to confuse chemical reactions with phase changes.

A phase change happens when matter changes form, such as:

• ice melting into liquid water
• water freezing into ice
• liquid water evaporating into water vapor

In these changes, the substance stays the same. Water is still water. So these are not examples of reactants turning into new products.

In a chemical reaction, the products are new substances. That is the big difference.

Quick Check

Try these on your own:

1. In the sentence $$\text{wood} + \text{oxygen} \rightarrow \text{ash} + \text{smoke}$$, what are the reactants?
2. What are the products?
3. If milk turns sour and becomes a different substance, which is the reactant: fresh milk or sour milk?
4. If a reaction gives off bubbles, what might that tell you?

Answers:

1. The reactants are wood and oxygen.
2. The products are ash and smoke.
3. Fresh milk is the reactant. Sour milk is the product.
4. It may tell you that a gas formed, which is a sign of a chemical reaction.

Summary

Reactants are the substances at the beginning of a chemical reaction.

Products are the new substances formed at the end of the reaction.

To identify them, remember: before the arrow = reactants and after the arrow = products.

When you map a chemical reaction, you track what starts the reaction, what happens during the change, and what new substances are made.

If new substances form, it is a chemical reaction. If the substance stays the same, like ice melting into water, it is a physical change, not a reactants-to-products chemical change.

Law of Conservation of Mass means that matter is not created and not destroyed. It can change form, change shape, or mix with other matter, but the total mass stays the same in a closed system.

A closed system is a setup where matter cannot get in or out. For example, if you seal materials inside a bag, bottle, or container, all the matter stays inside. This helps us prove that the amount of matter does not change during physical or chemical changes.

This idea is called the Law of Conservation of Mass. In simple words: what you start with must equal what you end with, as long as no matter escapes and no extra matter enters.

We can write the idea like this:

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

This law is important when we study changes in matter. Matter can go through physical changes and chemical changes, but its total mass still stays the same in a closed system.

Physical changes happen when matter changes size, shape, or state. For example, ice melts into liquid water. The water looks different, but it is still the same matter.

Chemical changes happen when substances react and form new substances. For example, vinegar and baking soda react to make new materials, including a gas. Even though new substances form, the total mass stays the same if all the matter is kept in a closed system.

Thermal energy transfer can also cause changes in matter. When matter gains or loses heat, it may melt, freeze, evaporate, or condense. These are changes of state, but the amount of matter does not disappear.

Sometimes people think mass changes because they cannot see all the matter. This often happens when a gas is made. Gas can spread out and seem to vanish, but it is still matter and still has mass.

For example, when water evaporates, it turns into water vapor, a gas in the air. The water did not get destroyed. It only changed state from liquid to gas.

To prove conservation of mass, scientists use closed-system experiments. In these experiments, they measure the mass before a change happens and then measure the mass again after the change. If the system is closed, the two masses should match.

Main Idea: If no matter can leave or enter, then the total mass stays the same during both physical and chemical changes.

• Matter is anything that has mass and takes up space.
• Mass is the amount of matter in an object.
• Closed system means matter cannot get in or out.
• Physical change means the matter changes form, but stays the same substance.
• Chemical change means new substances are formed.

Let’s look at physical changes first.

If you put an ice cube in a sealed container and let it melt, the solid ice becomes liquid water. The form changes, but the total mass in the sealed container stays the same.

If you heat water in a sealed container, some liquid water may become water vapor. Even though the liquid level changes, the total mass of the sealed container and everything inside it remains unchanged.

Now let’s look at chemical changes.

If baking soda and vinegar are mixed in an open cup, the reaction makes a gas that can escape into the air. If you only measure what is left in the cup, it may seem like mass was lost. But really, some matter left the cup as gas.

If the same reaction happens inside a sealed bag, the gas stays trapped. Then the total mass of the bag and everything in it will be the same before and after the reaction.

This is why closed systems are so important when we test the law of conservation of mass. They make sure all the matter is counted.

How to think about it:

1. Start with a certain amount of matter.
2. Let the matter change physically or chemically.
3. Keep all matter inside a closed system.
4. Measure total mass before and after.
5. The totals should be equal.

Here is a simple way to compare before and after:

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

That can also mean:

$$\text{mass of reactants} = \text{mass of products}$$

Reactants are the starting substances in a chemical reaction. Products are the new substances made after the reaction.

Worked Example 1: Melting Ice

A sealed container with ice inside has a mass of \(120\) grams. The ice melts into liquid water. What is the mass of the sealed container and water after melting?

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

Step 2: Check whether the system is closed. The container is sealed, so yes, it is a closed system.

Step 3: Use the law of conservation of mass.

$$120\text{ g before} = 120\text{ g after}$$

Answer: The mass after melting is 120 grams.

Worked Example 2: Water Evaporating in a Sealed Jar

A sealed jar with water in it has a mass of \(300\) grams. After sitting in the sun, some of the water evaporates into water vapor inside the jar. What is the total mass now?

Step 1: Evaporation is a physical change.

Step 2: The jar is sealed, so no matter escapes.

Step 3: The total mass stays the same.

$$300\text{ g before} = 300\text{ g after}$$

Answer: The total mass is still 300 grams.

Worked Example 3: Chemical Reaction in a Sealed Bag

A student places \(10\) grams of baking soda and \(25\) grams of vinegar in a sealed bag. The substances react and make bubbles of gas, but the gas stays inside the bag. What is the total mass after the reaction?

Step 1: Add the starting masses.

$$10\text{ g} + 25\text{ g} = 35\text{ g}$$

Step 2: The bag is sealed, so this is a closed system.

Step 3: Use conservation of mass.

$$\text{mass of products} = 35\text{ g}$$

Answer: The total mass after the reaction is 35 grams.

Worked Example 4: Why an Open System Can Trick Us

A cup holds \(50\) grams of a fizzy liquid. After some time, the cup and liquid measure \(47\) grams. Did matter get destroyed?

Step 1: Notice that the cup is open, not closed.

Step 2: Gas likely escaped into the air.

Step 3: The missing \(3\) grams did not get destroyed. It left the cup as gas.

We can show the change measured in the cup:

$$50\text{ g} - 47\text{ g} = 3\text{ g}$$

Answer: No, matter was not destroyed. Some matter escaped, so the cup was not a closed system.

What closed-system experiments show

• Mass stays the same when ice melts in a sealed container.
• Mass stays the same when water evaporates in a sealed container.
• Mass stays the same when a chemical reaction happens in a sealed bag or bottle.
• If mass seems to change, some matter probably entered or left the system.

Common mistakes to avoid

• Thinking matter disappears when it becomes a gas.
• Forgetting that a closed system must keep all matter inside.
• Mixing up physical changes and chemical changes.
• Looking only at what you can see instead of all the matter present.

Quick Check

Ask yourself these questions:

• Is the system closed?
• Did matter change state, like melting or evaporating?
• Did a chemical reaction make a gas?
• Was all the matter measured before and after?

If the answer is yes to the last question and the system is closed, then the total mass should stay the same.

Summary

The Law of Conservation of Mass says that matter is neither created nor destroyed. During physical changes and chemical changes, the total mass stays the same if the system is closed.

Closed-system experiments help us prove this law because they keep all matter inside. Even when matter melts, evaporates, or forms a gas in a reaction, the total mass before and after stays equal.

Conservation of Mass means that matter does not disappear and new matter does not pop out of nowhere.

When something changes, the total amount of matter stays the same, especially in a closed system. A closed system is a space where nothing can get in and nothing can get out.

This can sound tricky, but it is really a simple idea: the pieces may change shape, size, or mix together, but the total amount is still there.

Introduction

Everything around us is made of matter. Water is matter. Air is matter. A toy block is matter. Sand is matter.

Matter can change. Ice can melt into water. Paper can be cut into tiny pieces. Ingredients can be mixed together.

Even when matter changes, the total amount of matter stays the same if none of it escapes. That is conservation of mass.

Main Teaching Points

1. Matter is all the “stuff” things are made of.

• A rock is matter.
• Milk is matter.
• The air in a balloon is matter.

If something is made of “stuff” and takes up space, it is matter.

2. Matter can change in different ways.

• Physical change: Matter changes how it looks, but it is still the same kind of matter. Example: ice melting.
• Mixing: Two or more kinds of matter are put together. Example: sand and water mixed in a jar.

In these changes, the total amount of matter stays the same if all of it stays in the system.

3. Closed system means nothing gets in or out.

Imagine a jar with a lid on tight. If you put objects in the jar and close it, the matter is trapped inside. That helps us see that the total amount stays the same.

If some water spills out, then it is not a closed system anymore. If air escapes from a balloon, some matter leaves the system.

4. Changing shape does not change the amount.

If you break a cracker into 4 pieces, you still have the same cracker. The shape changed, but the total amount did not.

If you flatten a ball of clay, it is still the same amount of clay. It only looks different.

5. Mixing does not make matter disappear.

If you pour 1 cup of water and 1 cup of sand into a closed container, the matter is still there. It is mixed together, but it did not vanish.

The matter may be harder to separate, but the total amount is still the same.

Worked Examples

Example 1: Cutting an apple

A whole apple is cut into 6 slices.

Did the amount of matter change? No.

The apple changed shape, but all the pieces together are still the same apple.

We can think of it like this:

$$1\text{ apple} = 6\text{ slices of the same apple}$$

Example 2: Melting ice

You put 3 ice cubes in a cup. Later, the ice cubes melt into liquid water.

Did the amount of matter change? No.

The ice changed from solid ice to liquid water, but it is still the same amount of matter if none spills out.

We can write:

$$3\text{ ice cubes} \rightarrow \text{the same amount of water}$$

Example 3: Clay in a closed bag

You put a lump of clay in a sealed plastic bag. Then you press the clay flat.

Did the amount of matter change? No.

The clay changed shape, but no clay left the bag and no new clay came in. The bag is a closed system.

Example 4: Mixing water and juice powder

You pour water into a bottle. Then you add juice powder and close the bottle tightly. You shake it.

The powder seems to disappear, but did the matter disappear? No.

The powder mixed into the water. The matter is still in the bottle.

Before mixing, there was water and powder. After mixing, there is juice. The total amount of matter stays inside the closed bottle.

We can show the idea with a simple number sentence:

$$2 + 1 = 3$$

That means if you start with 2 parts of one thing and 1 part of another thing, you still have 3 parts of matter total.

How to Think About Conservation of Mass

1. Look at the matter at the start.
2. Ask: Did it change shape, melt, or mix?
3. Ask: Did any matter leave?
4. If nothing left and nothing was added, the total amount stays the same.

Important Things to Remember

• Matter cannot be created from nothing.
• Matter cannot be destroyed into nothing.
• Matter can change form, shape, or be mixed.
• In a closed system, the total amount of matter stays the same.

Try Thinking About These

1. A sheet of paper is torn into tiny pieces. Is there the same amount of matter? Yes.

2. A popsicle melts in a cup. Is there the same amount of matter if none spills? Yes.

3. Air leaks out of a balloon. Is it still a closed system? No. Some matter left.

Brief Summary

Conservation of mass means the total amount of matter stays the same in a closed system.

Matter can be cut, melted, mixed, or changed in shape, but it does not disappear.

If nothing gets in and nothing gets out, the amount of matter stays the same.

Combustion and Oxidation

Have you ever seen a candle burn, a campfire glow, or a bike left outside start to rust? These are all examples of chemical changes involving oxygen.

In this lesson, you will learn about two important kinds of changes: combustion and oxidation. Both happen when a substance reacts with oxygen, but they do not happen in the same way.

Combustion is a fast reaction with oxygen that gives off lots of heat and often light. We usually call this burning.

Oxidation is a more general word for a reaction with oxygen. Sometimes oxidation happens quickly, like in a fire. Sometimes it happens very slowly, like when iron rusts over days, weeks, or months.

So, combustion is a kind of oxidation, but not all oxidation is combustion.

Why does this matter? These changes help us understand how matter can be transformed into new substances. They also help explain where heat and light can come from during chemical reactions.

Main Idea 1: What is needed for combustion?

For combustion to happen, three things are usually needed:

• Fuel — something that can burn, like wood, paper, or candle wax
• Oxygen — usually from the air
• Heat — enough warmth to start the burning

If one of these is missing, combustion will stop or never begin.

For example, if you put a glass over a small candle flame, the flame goes out after a while. That happens because the candle uses up the oxygen inside the glass.

Main Idea 2: What happens during combustion?

During combustion, the fuel reacts with oxygen and forms new substances. Because new substances are made, combustion is a chemical change.

Combustion often gives off:

• Heat energy
• Light energy
• Gases such as carbon dioxide and water vapor
• Sometimes smoke or ash

For example, when wood burns, it changes into ash, gases, heat, and light. The wood cannot be changed back into the exact same piece of wood, so this is not a physical change. It is a chemical change.

Main Idea 3: What is oxidation?

Oxidation happens when a material reacts with oxygen. This can happen quickly or slowly.

Fast oxidation is combustion. Slow oxidation happens more gently and usually does not make flames.

One common example of slow oxidation is rusting. Rusting happens when iron reacts with oxygen, usually with water helping the process.

Rust is a new substance that forms on iron. It is often reddish-brown and flaky. A rusty nail is no longer exactly the same as a shiny new nail because a chemical change has happened.

Main Idea 4: Combustion and rusting are alike and different

Combustion and rusting are alike because:

• Both involve oxygen
• Both are chemical changes
• Both make new substances

They are different because:

• Combustion is fast; rusting is slow
• Combustion gives off a lot of heat and often light; rusting usually does not
• Combustion often has a flame; rusting does not

Main Idea 5: Signs of a chemical change

How can you tell that combustion or oxidation is a chemical change?

Look for clues such as:

• A new color appears
• A gas is produced
• Heat or light is released
• A new solid forms, such as rust or ash

These clues show that matter has changed into something new.

Main Idea 6: Thermal energy and chemical change

Combustion is closely connected to thermal energy. A fuel must first be heated enough to start burning. Then the reaction releases even more heat.

That heat can spread to nearby matter. This is why one burning log can help another log catch fire.

Rusting is different. It is also a chemical change involving oxygen, but it happens so slowly that the energy release is not easy to notice.

Worked Example 1: Is a candle burning an example of combustion or slow oxidation?

Question: A candle is lit and gives off heat and light. Is this combustion or slow oxidation?

Step 1: Ask if oxygen is involved. Yes, the flame uses oxygen from the air.

Step 2: Ask if the reaction is fast and gives off heat and light. Yes, it does both.

Answer: This is combustion, which is a fast kind of oxidation.

Worked Example 2: Is rust forming on a bike an example of combustion?

Question: A bike is left in the rain, and after many days, rust appears. Is this combustion?

Step 1: Check whether oxygen is involved. Yes, iron reacts with oxygen.

Step 2: Check whether it happens quickly with flames, heat, and light. No, it happens slowly and without flames.

Answer: This is oxidation, but not combustion. It is a slow chemical change called rusting.

Worked Example 3: What happens if oxygen is removed?

Question: A small flame is covered with a jar. Soon the flame goes out. Why?

Step 1: Remember that combustion needs fuel, oxygen, and heat.

Step 2: The jar traps only a limited amount of air inside.

Step 3: The flame uses the oxygen in that trapped air.

Answer: When the oxygen is mostly used up, combustion cannot continue, so the flame goes out.

Worked Example 4: Compare two changes

Question: One student burns a piece of paper. Another student notices a metal gate getting rusty over time. Which change releases energy faster?

Step 1: Burning paper is combustion.

Step 2: Rusting metal is slow oxidation.

Step 3: Combustion releases heat and light quickly, while rusting happens slowly.

Answer: Burning paper releases energy faster.

Helpful Comparison Chart

• Combustion: fast, uses oxygen, gives off lots of heat, often gives off light, may have flames
• Oxidation: reaction with oxygen, can be fast or slow
• Rusting: slow oxidation of iron, no flame, very little noticeable heat

Real-Life Examples

• A campfire burning wood — combustion
• A gas stove flame — combustion
• A candle flame — combustion
• A rusty nail — slow oxidation
• A rusty bike chain — slow oxidation

Safety Note

Combustion can be useful for cooking, heating, and giving light, but fire can also be dangerous. Always follow adult safety rules around flames, stoves, candles, and matches.

Summary

Combustion and oxidation are chemical changes that involve oxygen. Combustion is a fast reaction that releases lots of heat and often light. Oxidation can also happen slowly, like when iron rusts.

Both processes change matter into new substances. The big difference is the speed and how much energy is released in a short time.

Irreversible Chemical Changes

Everything around us is made of matter. Sometimes matter can change.

Some changes can be undone, but some changes cannot be undone. Today we will learn about a kind of change called an irreversible chemical change.

An irreversible chemical change is a change that makes something new, and you cannot change it back to the way it was before.

For first graders, we can say it like this: a big change happens, a new thing is made, and it cannot be changed back easily.

How can we tell if it is an irreversible chemical change?

• A new substance is made.

• The change is hard or impossible to undo.

• Sometimes we see clues like a new smell, a color change, heat, smoke, bubbles, or something getting hard after it was soft.

Important idea: Not every change is a chemical change. If you tear paper, melt ice, or break a crayon, it is still the same kind of matter. But when wood burns or cake bakes, new materials are made.

Examples of irreversible chemical changes

1. Burning wood: Wood changes into ash, smoke, and gases. You cannot turn the ash back into the same piece of wood.

2. Rusting iron: Iron can form rust. Rust is different from the shiny iron at the start. You cannot easily make it shiny iron again.

3. Baking a cake: The batter goes into the oven soft and wet. The cake comes out baked, fluffy, and different. You cannot turn the baked cake back into batter.

4. Cooking an egg: A raw egg changes when it is cooked. You cannot make it raw again.

Let’s think about what is happening.

Before the change, we start with one kind of material. After the change, we have something new.

We can show that idea like this:

start 6 new thing

This means the beginning material changes into a different material.

Worked Example 1: Burning wood

You see a log in a fire. Later, you see ash and smoke.

Question: Is this an irreversible chemical change?

Think: Did a new substance form? Can we change the ash back into the same log?

Answer: Yes. Burning wood is an irreversible chemical change because ash and smoke are new substances, and the log cannot be changed back.

Worked Example 2: Rust on a bike

A metal bike is left out in the rain for a long time. Orange-brown rust forms.

Question: Is rusting an irreversible chemical change?

Think: The metal changed and made rust. Rust is different from the metal at the start.

Answer: Yes. Rusting is an irreversible chemical change because a new substance forms, and it cannot easily go back to the way it was before.

Worked Example 3: Baking a cake

You mix eggs, flour, and milk to make batter. Then you bake it in the oven. It becomes cake.

Question: Is baking a cake an irreversible chemical change?

Think: The cake is not the same as the wet batter. It smells different, looks different, and feels different.

Answer: Yes. Baking a cake is an irreversible chemical change because baking makes a new substance, and you cannot turn the cake back into batter.

Worked Example 4: Melting ice

An ice cube sits on a table and melts into water.

Question: Is this an irreversible chemical change?

Think: Is it still the same material? Can it go back?

Answer: No. Melting ice is not an irreversible chemical change. It is still water, and it can freeze again.

Let’s compare

• Burning wood 6 ash and smoke: new substances are made.

• Baking cake batter 6 cake: a new substance is made.

• Rusting iron 6 rust: a new substance is made.

• Melting ice 6 water: no new substance is made.

Clues to remember

• Ask, “Did it make something new?”

• Ask, “Can it go back to the way it was before?”

• If something new is made and it cannot go back, it is likely an irreversible chemical change.

Try it in your mind

If bread dough is baked into bread, is it an irreversible chemical change?

Yes. The dough becomes bread, which is a new substance, and it cannot be changed back into dough.

If paper is cut into smaller pieces, is it an irreversible chemical change?

No. The paper is still paper. It changed shape, but no new substance was made.

Safety note

Some chemical changes, like burning, can be dangerous. Always let a grown-up handle fire, ovens, and hot things.

Summary

An irreversible chemical change happens when matter changes and makes a new substance.

You usually cannot change it back to what it was before.

Burning wood, rusting iron, baking a cake, and cooking an egg are all examples of irreversible chemical changes.

To decide if a change is an irreversible chemical change, ask: Did it make something new? Can it go back?

Chemical Changes: Indicators and Evidence

Everything around us is made of matter. Matter can change in different ways. Some changes are physical changes, and some are chemical changes.

A physical change is when something looks different but is still the same kind of matter. For example, ice melting into water is a physical change. It changes shape and state, but it is still water.

A chemical change is when matter changes into a new substance. The new substance has different properties from what we started with. That means a chemical change makes something new.

We cannot always see tiny particles, but we can look for evidence. Evidence is a clue that helps us know a chemical change happened.

Main Signs of a Chemical Change

• Gas is produced — You may see bubbles or fizzing.
• Temperature changes on its own — It gets warmer or cooler without heating or cooling it on purpose.
• Color changes — A new color appears.
• A solid forms in a liquid — This new solid is called a precipitate.

Let’s learn about each sign.

1. Gas is produced

Sometimes, when two materials are mixed, they make a gas. The gas may look like bubbles, foam, or fizz.

For example, if vinegar and baking soda are mixed, lots of bubbles appear. Those bubbles are evidence that a new substance is being made.

Be careful: bubbles do not always mean a chemical change. Water boiling makes bubbles too, but that is a physical change because the water is still water. So we have to think carefully about what is happening.

2. Temperature changes without being heated or cooled

Sometimes a mixture gets warmer or colder all by itself. If no one heated it with a stove or cooled it with ice, that change can be evidence of a chemical change.

For example, some reactions make heat. Others take in heat and feel colder. A temperature change can be a clue that new substances formed.

3. Color changes

A color change can be another sign of a chemical change. If one material turns into a different color after being mixed with something else, that may be evidence that a new substance formed.

For example, when iron rusts, it changes from gray to reddish-brown. Rust is a new substance, so rusting is a chemical change.

4. A solid forms in a liquid

Sometimes two liquids are mixed, and then a new solid appears. That solid is called a precipitate.

If a solid suddenly forms after two liquids are mixed, that is strong evidence of a chemical change. The new solid was not there before.

Why These Signs Matter

These signs help us act like scientists. We observe carefully and look for clues. If we notice gas, a color change, a temperature change, or a new solid, we can say there is evidence of a chemical change.

Sometimes one sign appears. Sometimes more than one sign appears at the same time. The more evidence we have, the more sure we can be.

Chemical Change or Physical Change?

It is important to tell the difference.

• Physical change: No new substance is made.
• Chemical change: A new substance is made.

Here are some physical changes:

• Ice melting
• Water freezing
• Tearing paper
• Breaking a crayon

Here are some chemical changes:

• Rust forming on iron
• Baking a cake
• Wood burning
• Vinegar and baking soda fizzing

Worked Example 1

Question: A student mixes vinegar and baking soda. The cup starts to fizz and bubble. Is this evidence of a chemical change?

Think: One sign of a chemical change is gas being produced.

Answer: Yes. The fizzing and bubbles are evidence that gas is being made. That means a chemical change is happening.

Worked Example 2

Question: An ice cube melts and becomes liquid water. Is this a chemical change?

Think: Did a new substance form? No. It was water as a solid, and it is still water as a liquid.

Answer: No. This is a physical change, not a chemical change.

Worked Example 3

Question: A shiny metal bike left outside becomes reddish-brown after a long time. Is this a chemical change?

Think: A color change happened, and rust formed. Rust is a new substance.

Answer: Yes. This is a chemical change because a new substance formed.

Worked Example 4

Question: Two clear liquids are mixed. After a minute, a cloudy solid appears at the bottom. What evidence shows a chemical change?

Think: A new solid forming in a liquid is called a precipitate.

Answer: The evidence is that a solid formed. That shows a chemical change may have happened.

Tips for Finding Evidence

1. Look closely at what happens after materials are mixed.
2. Ask: Do I see bubbles or fizzing?
3. Ask: Did the temperature change by itself?
4. Ask: Did the color change?
5. Ask: Did a new solid appear?
6. Decide if these clues show that a new substance formed.

Remember

Not every change is a chemical change. If something only changes size, shape, or state, it is usually a physical change.

But if the change makes a new substance, it is a chemical change. We know this by looking for evidence.

Brief Summary

A chemical change happens when matter forms a new substance. Scientists look for evidence such as gas being produced, temperature changing on its own, color changing, and a new solid forming. These clues help us tell chemical changes from physical changes.

Conservation of Mass means that matter does not disappear and new matter does not just appear from nowhere.

When matter changes, the total mass stays the same. This is true when matter changes its shape, changes its state, or mixes with other matter.

Mass is how much matter is in something. We can measure mass with a balance or scale.

So the big science idea is this: matter cannot be created or destroyed. It can only change form.

Here is another way to say it:

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

This is called the conservation of mass.

Why does this happen?

All matter is made of tiny pieces called particles. We may not be able to see them, but they are there.

When matter changes, the particles can move, spread out, mix, melt, freeze, or change into a gas. But the particles are still matter.

That is why the total mass stays the same.

Changes that still keep the same mass

Many kinds of changes follow conservation of mass.

• Changing shape: A clay ball rolled into a snake shape still has the same mass.
• Changing size: A cracker broken into pieces still has the same total mass.
• Changing state: Ice melting into liquid water still has the same mass.
• Mixing: Sand mixed with water has the same total mass as the sand and water before mixing.

Important idea: Nothing is lost

Sometimes it can look like matter is gone. But often it has just moved somewhere else.

For example, if a puddle dries up, the water did not get destroyed. It changed into water vapor in the air.

If you could measure all of the water before and after, the mass would still match.

Think about closed and open containers

A closed container is sealed. Matter stays inside.

An open container is not sealed. Some matter, like a gas, can move out into the air.

If matter leaves an open container, it may look like mass was lost. But really, the matter just moved away from where we were measuring.

That is why scientists often use closed containers when they test conservation of mass.

Worked Example 1: Breaking matter into pieces

A granola bar has a mass of 30 grams. A student breaks it into 3 pieces.

What is the total mass of the 3 pieces?

Step 1: Breaking something does not destroy matter.

Step 2: Add the masses of all the pieces together. They must equal the starting mass.

$$30 = 30$$

Answer: The total mass of the 3 pieces is 30 grams.

Worked Example 2: Melting ice

An ice cube has a mass of 12 grams. It melts into liquid water.

What is the mass of the water after melting?

Step 1: Melting is a change of state.

Step 2: The matter is still the same water, so the mass stays the same.

$$12 = 12$$

Answer: The water has a mass of 12 grams.

Worked Example 3: Mixing two kinds of matter

A cup holds 20 grams of water. Then 5 grams of sugar are added and mixed in.

What is the total mass after mixing?

Step 1: Add the mass of the water and the mass of the sugar.

$$20 + 5 = 25$$

Step 2: Mixing does not destroy matter.

Answer: The mixture has a total mass of 25 grams.

Worked Example 4: When matter seems to disappear

A bowl has 50 grams of soup. Some of the water in the soup changes into gas and rises into the air because the bowl is left open.

When the bowl is measured later, the soup in the bowl has a mass of 46 grams.

Did matter get destroyed?

Step 1: Compare the masses in the bowl.

$$50 - 46 = 4$$

Step 2: The 4 grams did not get destroyed. Some matter left the open bowl and moved into the air as gas.

Answer: No, matter was not destroyed. It changed form and moved out of the bowl.

How to remember conservation of mass

• Same matter, same total mass.
• Changes do not mean matter is gone.
• If matter moves away, it may look missing, but it still exists.

Try thinking about these situations

1. A piece of paper is cut into small squares. Does the total mass change?
2. Juice is poured from a tall cup into a short cup. Does the mass change?
3. An ice pop melts. Does the mass change?
4. Water sits in an open cup for days and some changes into gas. Is the matter destroyed?

The answers are: the total mass stays the same in the first three situations. In the last situation, the matter is not destroyed; some of it moves into the air.

Brief Summary

Conservation of mass means the total mass of matter stays the same, even when matter changes shape, size, state, or gets mixed.

Matter cannot be created or destroyed. It can only change form or move from one place to another.

When we measure all the matter before and after a change, the total mass matches.