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PARTICLE MODEL OF MATTER · CHAPTER 4 OF 10

Changes of state

A frozen smoothie melts, a wet hoodie dries and droplets appear on a cold drinks bottle. These changes look different, but each involves a substance changing state.

By the end, you’ll be able to…

  • Name and describe the main changes of state.
  • Explain how evaporation differs from boiling.
  • Explain why mass is conserved during a change of state.
  • Distinguish a change of state from a chemical change.

Same substance, different state

A change of state happens when a substance changes between solid, liquid and gas. Its particles remain the same: their arrangement and movement change. An ice lolly melts, a wet hoodie dries and a bathroom mirror mists over. These changes look different, but each involves a substance changing state.

A melting ice lolly

Solid ice changes into liquid water.

A drying hoodie

Liquid water evaporates into the surrounding air.

Dots represent water molecules. Water vapour is invisible in real life.

A misted bathroom mirror

Water vapour in the air condenses into tiny liquid droplets on the cooler mirror.

The water comes from the surrounding air. Each visible droplet contains many water molecules.
Simplified models, not to scale. Molecule symbols are enlarged; real gas molecules move randomly. Changes are sped up.

Naming the changes

Each direction has its own name. Heating can cause melting, boiling or sublimation. Cooling can cause freezing or condensation.

Solid → liquid = Melting

Liquid → solid = Freezing

Liquid → gas = Boiling or evaporation

Gas → liquid = Condensation

Solid → gas = Sublimation

Gas → solid = Deposition

Sublimation happens without a liquid stage. For example, solid carbon dioxide—dry ice —changes directly into carbon dioxide gas. The reverse change, gas directly to solid, is called deposition.

INTERACTIVE EXPLORER

What changes inside?

These models represent the same sample in three states. Predict what changes—and what stays the same—before selecting a state.

Compare the number, size, arrangement and movement of the particles.
12 particle symbols

The particles are close together in a regular arrangement. They vibrate about fixed positions.

During melting, the particles become able to move past one another. The particles themselves do not melt.

12 particle symbols

The particles remain close together, but their arrangement is irregular. They can move past one another.

During freezing, the particles settle into fixed positions but continue to vibrate. When the liquid becomes a gas, its particles become much more widely separated.

12 particle symbols

The particles are widely separated and move randomly throughout the available space.

During condensation, particles come close together to form a liquid. They remain the same particles.

Which features stayed the same?

The particle type, size and number stayed the same. Their arrangement and movement changed.

No particles entered or left the sample, so its mass stayed the same.

Simplified model of a generic substance, not to scale. Motion is illustrative; liquid particles do not move in organised rows in real life. Water is unusual: its particles are arranged further apart in ice than in liquid water.

Evaporation and boiling: what is the difference?

Both change a liquid into a gas, but they happen differently. A wet hoodie can dry in a cool room without its water boiling.

Evaporation

Happens at the surface of a liquid. Some particles escape into the surrounding space.

It can happen at temperatures below the boiling point. No bubbles need to form within the liquid.

Boiling

Happens throughout the liquid. Bubbles of the substance’s gas form inside it and rise.

It occurs at the boiling point, which depends on the surrounding pressure.

Pure water boils at about 100°C at normal atmospheric pressure. It can evaporate at room temperature. Water vapour is invisible; the white cloud near a kettle is made of tiny liquid droplets formed by condensation.

Sublimation happens without a liquid stage. For example, solid carbon dioxide—dry ice—changes directly into carbon dioxide gas. The reverse change, gas directly to solid, is called deposition.

INTERACTIVE EXPLORER

Does water have to boil to become a gas?

A damp makeup sponge is left in a room at 20°C. A saucepan of pasta water is boiling at 100°C.

Which can change liquid water into water vapour?

Make a prediction. Choose an answer to start both animations.
Water can evaporate from the sponge, but boiling water also becomes water vapour. Watch both examples, then try again.
Water does not need to reach its boiling point to become a gas. It can evaporate from the damp sponge at room temperature too.
Correct—both! Water evaporates from the sponge’s exposed surfaces. In the saucepan, water-vapour bubbles form throughout the boiling liquid and rise to the surface.

A damp makeup sponge

Room temperature: 20°C

Evaporation: water molecules escape from exposed liquid surfaces into the air. No boiling bubbles form.

The wet patch fades as water leaves the sponge.

A saucepan of pasta water

Boiling: 100°C*

Boiling: water-vapour bubbles form throughout the liquid, rise and burst at the surface.

*About 100°C at normal atmospheric pressure. The pan is shown as a cutaway.
Animations are sped up. Dots represent individual water molecules, enlarged to make them visible. Each boiling bubble contains many water molecules. Water vapour itself is invisible.

Does changing state change the mass?

Mass is conserved during a change of state. No particles are created or destroyed. If none enter or leave the sample, its mass stays the same.

A sealed container makes this easier to check because it keeps all the substance inside, including any gas.

INTERACTIVE EXPLORER

Will the balance reading change?

A sealed jar containing ice is on a balance. The jar and its contents have a total mass of 180 g.

What will the balance read after all the ice melts?

The lid stays closed. No substance enters or leaves. Choose an answer to watch the ice melt.
Melting does not remove any water molecules. The sealed jar keeps all its contents, so the total mass does not decrease. Watch the reading, then try again.
Correct—the reading stays at 180 g. The same water molecules remain inside the jar. Changing their arrangement does not change the total mass.
No extra substance enters the jar. Melting changes the water’s state, but it does not add water molecules or increase the total mass.
Lid stays closed 180 g
The balance measures the jar, lid and everything inside.
Total mass 180 g

Look at the closed lid. Where could any of the water go?

The ice melts, but the balance reading stays the same.

No water molecules are created, destroyed or lost from the jar. Mass is conserved.

Could the water’s volume change while its mass stays the same?

Yes. Ice occupies more volume than the same mass of liquid water. When the ice melts, the water occupies less volume.

Its molecules are arranged differently, but the same amount of water remains. A change in volume does not necessarily mean a change in mass.

Idealised model, not to scale. Melting is sped up. Energy transfers from the warmer surroundings into the jar. The balance reading is treated as constant.

Why can the measured mass decrease?

If water evaporates from an open container, some water molecules leave and mix with the air. A balance weighing the container and remaining water then reads less.

The missing mass has been transferred to the surroundings as water vapour. It has not been destroyed. The total mass is conserved when the escaped water is included.

A physical change, not a new substance

A change of state is a physical change. No new substance forms, and reversing the change restores the substance’s original properties under the same conditions. Liquid water can freeze back into ice.

A chemical change produces new substances. Candle wax melting is a physical change; candle wax burning is a chemical change.

INTERACTIVE QUICK CHECK

Choose one answer for each question. You’ll see feedback straight away.

1. A damp makeup sponge dries at room temperature. What happens to the water?
2. Solid carbon dioxide changes directly into carbon dioxide gas. What is this change called?
3. Ice melts inside a sealed jar. Which statement explains why this is a physical change?
INTERACTIVE CHALLENGE

A student leaves an open dish of water on a balance. The reading decreases as the water evaporates. She says, “This proves mass is lost during a change of state.” Explain why her conclusion is incorrect.

Explain where the water goes and how you could change the setup to keep all the water on the balance.

Water molecules leave the dish as water vapour and mix with the surrounding air. The balance no longer measures those molecules, so its reading decreases.

The water has moved into the surroundings; it has not been destroyed. The total mass of the dish, remaining water and escaped water vapour is conserved.

Use a sealed container and weigh the whole container and its contents. Water can still evaporate inside, but none escapes, so the total balance reading stays the same.

CHAPTER 4

Chapter summary

  • Melting changes solid to liquid;
    freezing changes liquid to solid.
  • Boiling and evaporation change liquid to gas;
    condensation changes gas to liquid.
  • Sublimation changes solid directly to gas,
    without a liquid stage. The reverse change is deposition.
  • Evaporation happens at a liquid’s surface and can occur below
    its boiling point. Boiling happens throughout the liquid,
    with bubbles of the substance’s gas forming inside it.
  • During a change of state, the particles’ arrangement and
    movement change. The particles themselves retain their
    identity and size.
  • Mass is conserved. A sealed sample keeps the
    same mass. If substance escapes from an open container,
    its balance reading can decrease, but the escaped substance
    still has mass.
  • Changes of state are physical changes.
    No new substance forms. Reversing the change restores the
    substance’s original properties under the same conditions.