Changes of State
Master Changes of State for GCSE Physics with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Changes of state are physical changes in which mass is conserved and the substance can return to its original form.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
When a substance changes state, the mass is conserved — no particles are lost or gained. Only the arrangement and energy of the particles change.
Changes of state are physical changes, not chemical ones. This is important because the material recovers its original properties if the change is reversed. Melting ice and refreezing the water gives back ice with identical properties.
The changes are melting, solid to liquid; evaporating or boiling, liquid to gas; condensing, gas to liquid; freezing, liquid to solid; and sublimation, solid directly to gas. Energy is transferred to the substance when it melts or boils, and away from it when it condenses or freezes.
Revision notes
Mass is conserved
When a substance changes state, no particles are lost or gained, so the mass stays the same.
Only the arrangement of the particles and their energy change. A sealed container of ice has the same mass after the ice melts.
A physical change
Changes of state are physical, not chemical.
The material recovers its original properties if the change is reversed. This distinguishes them from chemical changes, where new substances are formed and the change is usually not easily reversed.
The named changes
Melting: solid to liquid. Evaporating or boiling: liquid to gas. Sublimation: solid directly to gas.
Condensing: gas to liquid. Freezing: liquid to solid. Energy is transferred to the substance for the first group and away from it for the second.
Key points
- Mass is conserved during a change of state.
- Changes of state are physical, not chemical.
- The material recovers its original properties if reversed.
- Melting is solid to liquid.
- Condensing is gas to liquid.
- Sublimation is solid directly to gas.
Worked examples
Example 1
Explain why the mass of a substance does not change when it melts. [2 marks]
Model answer
No particles are lost or gained during a change of state (1 mark); only the arrangement and energy of the particles change, so the mass stays the same (1 mark).
Example 2
Explain why a change of state is described as a physical change rather than a chemical one. [2 marks]
Model answer
No new substance is formed (1 mark), and the material recovers its original properties if the change is reversed (1 mark).
Example 3
Name the change of state in which a solid becomes a gas directly. [1 mark]
Model answer
Sublimation (1 mark).
Common mistakes
Saying mass decreases when a substance evaporates.
In a sealed system the mass is conserved; the particles have simply spread out.
Calling a change of state a chemical change.
It is physical — no new substance is formed.
Confusing condensing and freezing.
Condensing is gas to liquid; freezing is liquid to solid.
Forgetting sublimation.
It is the solid to gas change and is examinable.
Exam tips
- Say no particles are lost or gained to explain mass conservation.
- Use the recovery of original properties to justify physical change.
- Learn all five named changes.
- State whether energy is transferred to or from the substance.
Key terms
- Change of state
- A physical change between solid, liquid and gas.
- Sublimation
- A solid changing directly to a gas.
- Physical change
- A change forming no new substance.
- Conserved
- Unchanged in total amount.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.