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Physics · Particle model of matter

Changes of state

Name the changes of state, describe how mass is conserved, and distinguish physical changes from chemical changes.

  • 6 key terms
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Teacher resources

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Student handouts

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Warm-up

Answer each one, then check.

  1. 1

    What is the change from solid to liquid called?

    Show answerHide answer

    Melting

  2. 2

    What is the change from gas to liquid called?

    Show answerHide answer

    Condensing

  3. 3

    What is the unit of mass?

    Show answerHide answer

    Kilogram

  4. 4

    What is freezing?

    Show answerHide answer

    Liquid to solid

  5. 5

    Give an example of a chemical change.

    Show answerHide answer

    Burning wood

Learning Objectives

  1. 1Name each change of state.
  2. 2Explain that mass is conserved in a change of state.
  3. 3Explain that changes of state are physical changes and are reversible.
  4. 4Draw and interpret particle diagrams for solids, liquids and gases.

CHANGES OF STATE

Changes of state are physical changes: the material recovers its original properties if the change is reversed. Mass is conserved.

The particles are the same before and after, so the mass is the same. Only the arrangement and energy of the particles change.

The Changes

Learn the names.

  • Melting

    From: Solid. To: Liquid. Energy: Energy taken in

  • Freezing

    From: Liquid. To: Solid. Energy: Energy given out

  • Boiling or evaporating

    From: Liquid. To: Gas. Energy: Energy taken in

  • Condensing

    From: Gas. To: Liquid. Energy: Energy given out

  • Sublimation

    From: Solid. To: Gas. Energy: Energy taken in

Mass in a Change of State

A 200 g ice cube melts to form water. What is the mass of the water? Explain.

Show the solutionHide the solution
  1. 1 Number of particles The same particles are present as liquid water
  2. 2 Mass Unchanged, because no particles are lost

Answer200 g: mass is conserved.

Physical or Chemical?

Explain why melting ice is a physical change but burning wood is a chemical change.

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  1. 1 Melting ice The water is the same substance, and it freezes back to ice with the same properties
  2. 2 Burning wood New substances are made and the wood cannot be recovered

AnswerPhysical changes are reversible and the material recovers its original properties.

Particle Explanation

Say what the particles do.

  • Melting

    Particles gain energy and break free from their fixed positions.

  • Boiling

    Particles gain enough energy to overcome the attractions and move far apart.

  • Condensing and freezing

    Particles lose energy and get closer.

Name the Change

Name each change: (a) water becomes steam; (b) steam becomes water on a cold window; (c) a puddle freezes; (d) dry ice becomes carbon dioxide gas without becoming liquid.

1. Name the start and end state.

2. Choose the change.

A good answer shows: (a) Boiling (or evaporating). (b) Condensing. (c) Freezing. (d) Sublimation.

Can I...?

  1. 1Name all the changes of state.
  2. 2Say whether energy is taken in or given out.
  3. 3Explain that mass is conserved.
  4. 4Explain physical changes.
  5. 5Draw solid, liquid, gas particles.
  6. 6Explain melting with particles.
  7. 7Distinguish physical and chemical.
  8. 8Give an example of sublimation.

Summary & Exam Focus

  • Melting, freezing, boiling, condensing, sublimation.
  • Mass is conserved.
  • Physical changes are reversible.
  • Particles' energy changes but the particles stay the same.

Exam focus

Name the change of state when a liquid becomes a solid. (1 mark) (1 marks)

Freezing.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Melting
Solid to liquid.
Freezing
Liquid to solid.
Boiling
Liquid to gas throughout the liquid.
Evaporating
Liquid to gas at the surface.
Condensing
Gas to liquid.
Sublimation
Solid straight to gas.

Questions and answers

10 questions set on this lesson, with the mark schemes and model answers open.

1. Exam question Complete 3 marks Easier

Name the change of state when (a) a solid changes to a liquid (b) a gas changes to a liquid (c) a solid changes to a gas without becoming a liquid.

Mark scheme — 3 marks available

  • Melting — 1 mark
  • Condensing — 1 mark
  • Sublimation — 1 mark

Model answer

(a) melting (b) condensing (c) sublimation

2. Exam question Explain 2 marks Easier

A 200 g ice cube melts. State the mass of the water and explain your answer.

Mark scheme — 2 marks available

  • 200 g — 1 mark
  • Mass conserved or the same number of particles — 1 mark

Model answer

200 g. Mass is conserved because the particles are the same; only their arrangement has changed.

3. Exam question Explain 2 marks Easier

Explain the difference between a physical change and a chemical change.

Mark scheme — 2 marks available

  • Physical: reversible with original properties — 1 mark
  • Chemical: new substances — 1 mark

Model answer

In a physical change the material can recover its original properties if the change is reversed. In a chemical change new substances are made and this cannot easily be reversed.

4. Exam question Describe 3 marks Easier

Describe what happens to the arrangement and movement of the particles when a solid melts.

Mark scheme — 3 marks available

  • Solid: fixed regular positions, vibrating — 1 mark
  • Gain energy — 1 mark
  • Liquid: still close but move around each other — 1 mark

Model answer

In a solid the particles are in fixed positions and vibrate. As the solid melts the particles gain energy, break away from fixed positions and can move around each other while staying close.

5. Exam question Explain 3 marks Easier

A student heats water until it boils. The water changes to steam. Explain, in terms of particles, why the steam takes up much more space than the water.

Mark scheme — 3 marks available

  • Particles far apart in gas — 1 mark
  • Close together in liquid — 1 mark
  • Same mass, greater volume — 1 mark

Model answer

In the steam the particles are far apart, but in the liquid they are close together. The same particles occupy a much larger volume, although the mass has not changed.

6. Multiple choice 1 mark Easier

Liquid to gas is called...

  1. A melting
  2. B freezing
  3. C boiling or evaporating Correct
  4. D condensing

Why: The liquid gains energy and becomes gas.

7. Multiple choice 1 mark Core

When ice melts, its mass...

  1. A increases
  2. B decreases
  3. C becomes zero
  4. D stays the same Correct

Why: Mass is conserved.

8. Multiple choice 1 mark Core

Melting is a...

  1. A physical change Correct
  2. B chemical change
  3. C nuclear change
  4. D electrical change

Why: The material recovers its properties if the change is reversed.

9. Multiple choice 1 mark Core

Condensing is when a...

  1. A liquid becomes a gas
  2. B gas becomes a liquid Correct
  3. C solid becomes a liquid
  4. D liquid becomes a solid

Why: Cooling a gas gives a liquid.

10. Multiple choice 1 mark Stretch

Sublimation is when a solid...

  1. A melts
  2. B freezes
  3. C changes directly to a gas Correct
  4. D condenses

Why: It skips the liquid state.