AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Particle model of matter › Changes of state and the particle model
Practise Changes of state. 8 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
Naming the changes of state, describing what happens to the particles, and knowing that mass is conserved because a change of state is a physical change. Questions are usually short recall and 2 to 4 mark explanations, often linked to particle diagrams or heating and cooling graphs.
Key facts
Melting: solid → liquid; freezing: liquid → solid
Boiling or evaporating: liquid → gas; condensing: gas → liquid
Sublimating: solid → gas directly
Mass is conserved in a change of state (same number of particles)
Change of state = physical change: reversible, and the material recovers its original properties
Chemical change: a new substance is formed
Volume and density change in a change of state; mass does not
Boiling: throughout the liquid, at the boiling point. Evaporation: at the surface, also below the boiling point
Boiling and evaporating: liquid → gas. Boiling happens throughout the liquid at its boiling point; evaporation happens only at the surface and can happen at temperatures below the boiling point.
Condensing: gas → liquid.
Sublimating: solid → gas directly, without becoming a liquid first. Example: solid carbon dioxide (dry ice).
The changes of state. Mass is conserved in every one.
A pure substance melts (and freezes) at a fixed temperature, its melting point, and boils at a fixed temperature, its boiling point.
What happens to the particles
Melting: energy supplied overcomes some of the forces holding the particles in fixed positions. They can now move past each other, but they stay close together.
Boiling: energy supplied overcomes the forces between the particles, so they move far apart and move quickly in all directions.
Freezing and condensing are the reverse: energy is transferred from the substance to the surroundings, and the particles become more closely held together.
The particles themselves do not change. Only their arrangement, spacing and motion change.
Mass is conserved: a physical change
When a substance changes state, its mass is conserved, because the number of particles stays the same.
The volume usually changes (the particles move closer together or further apart), so the density changes, but the mass does not.
A change of state is a physical change: if you reverse it, the material recovers its original properties. Ice that melts and then refreezes is still ice, made of the same water molecules.
In a chemical change a new substance is made (e.g. when fuel burns), and reversing the conditions does not give you the original material back.
If a liquid evaporates or boils in an open container, the mass of the container and its contents falls because particles escape into the air. Mass is still conserved: the missing mass is now in the air as gas.
How to answer each type of question
Name the change of state
1 mark eachGrade 3
Identify the starting state and the final state.
Use the exact term: melting, freezing, boiling (or evaporating), condensing or sublimating.
Example. Name the change of state in each case. (a) Water vapour forms droplets on a cold window. (b) A block of solid carbon dioxide turns straight into a gas. (c) Molten wax in a mould becomes solid.
State clearly what happens to the mass: it stays the same.
Give the reason: the number of particles does not change (no particles are added or removed).
Example. A student seals 40 g of ice in a plastic bag and leaves it until all the ice has melted. State the mass of water in the bag. Give a reason for your answer.
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40 g (1). No particles can enter or leave the sealed bag, so the number of particles stays the same and mass is conserved (1).
Explain why a change of state is a physical change
2 marksGrade 5
Say that no new substance is formed.
Say that the change can be reversed and the material recovers its original properties.
Example. Candle wax can be melted and then left to cool until it is solid again. Explain why melting wax is a physical change and not a chemical change.
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No new substance is formed when the wax melts (1). When the wax cools and freezes, it recovers its original properties, so the change is reversible (1).
Describe how the particles change
3 marksGrade 5
Describe the particles before the change: arrangement, spacing and motion.
Describe what is different after the change.
Use comparisons: 'regular' and 'random', 'fixed positions' and 'move past each other'.
Example. Describe how the arrangement and motion of the particles change when a solid melts to form a liquid.
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Any three from: in the solid, the particles are in a regular arrangement (1) in the solid, the particles vibrate about fixed positions (1) in the liquid, the arrangement is random (1) in the liquid, the particles can move past each other (1) the particles stay close together (1)
Don’t lose marks
Saying the mass changes when ice melts because the volume changes. The volume and density change; the mass does not.
Mixing up condensing (gas → liquid) and evaporating (liquid → gas).
Writing that the particles expand, melt or get bigger. Only their arrangement, spacing and motion change.
Saying a change of state is physical 'because it is reversible' without saying that no new substance forms or that the material recovers its original properties.
Treating boiling and evaporating as identical: evaporation happens at the surface, also at temperatures below the boiling point.
More tips
Memory tricks
Sublimation skips a step: solid → gas without passing through the liquid state.
Physical change = same substance, different state. Chemical change = new substance.
Sealed container: mass stays the same. Open container: mass can seem to change as particles escape into the air or join from it.
Opposite pairs: melting ↔ freezing, boiling or evaporating ↔ condensing.
Exam technique
Use the exact names of the changes of state; phrases such as 'turns into vapour' may not get the mark.
For conservation of mass, always give the particle reason: the number of particles stays the same.
When describing particles, cover arrangement (regular or random), spacing (close or far apart) and motion (vibrate, move past each other, move quickly in all directions).
If a question mentions an open container, think about particles escaping into, or coming from, the air.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name the changes of stateMelting, freezing, boiling, evaporating, condensing and sublimating.
Grade 4
State that mass is conservedWhen a substance changes state the number of particles stays the same, so its mass does not change.
Grade 5
Describe particle changes during melting and boilingDescribe how the arrangement, spacing and motion of the particles change.
Grade 5
Explain why a change of state is physicalNo new substance is made: if the change is reversed, the material recovers its original properties.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Solid carbon dioxide is sometimes called ‘dry ice’. At room temperature, dry ice changes directly from a solid into a gas. What is the name of this change of state?
Sublimation
Name the change of state when ice changes directly into water vapour.
Sublimation
A pure substance melts at 63 °C. At what temperature will the same substance freeze?
63 °C
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Solid carbon dioxide is sometimes called ‘dry ice’. At room temperature, dry ice changes directly from a solid into a gas. What is the name of this change of state?[1]
(b) Which change of state happens when water vapour turns into liquid water? Tick (✓) one box.[1]
Boiling
Condensing
Freezing
Melting
(c) A student put 45 g of ice into a plastic bag and sealed the bag. After one hour, all of the ice had melted. What was the mass of the water in the bag?[1]
(d) Give the reason for your answer.[1]
Show the answer and mark scheme
(a)Answer: Sublimation
sublimation / subliming / sublimating
(b)Answer: Condensing
(c)Answer: 45 g
45 (g)
(d)Answer: Mass is conserved in a change of state; the bag was sealed, so no particles could enter or leave.
mass is conserved when a substance changes state / no particles were added or removed because the bag was sealed
Question 2Medium6 marks
Melting, freezing, boiling, evaporating, condensing and sublimating are all changes of state.
(a) Changes of state are physical changes. Explain how a physical change is different from a chemical change.[2]
(b) A student boiled 250 g of water in an open beaker. After 5 minutes, the mass of water in the beaker was 212 g. The student said: ‘This shows that mass is not conserved when water boils.’ Explain why the student is wrong.[2]
(c) Describe how the arrangement and the movement of the particles change when a solid melts.[2]
Show the answer and mark scheme
(a)Answer: If a physical change is reversed, the material recovers its original properties; a chemical change produces a new substance with different properties.
in a physical change the material recovers its original properties if the change is reversed
in a chemical change a new substance (with different properties) is made
(b)Answer: The missing 38 g became steam that escaped into the air; the total mass of water plus water vapour is unchanged.
38 g of water turned into water vapour / steam which escaped from the open beaker (into the air)
the total mass of the water and the water vapour did not change / no particles were destroyed
(c)Answer: The particles go from a regular arrangement, vibrating about fixed positions, to a random arrangement in which they are still close together but can move past each other.
the arrangement changes from regular to random / irregular (the particles stay close together)
the particles change from vibrating about fixed positions to moving around / sliding past each other