A student says that boiling is a physical change, not a chemical change. Explain why the student is correct, using ideas about particles.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Boiling is a physical change because it is a change of state from liquid to gas. The chemical composition of the substance does not change – the same molecules are present before and after boiling. Energy is supplied to overcome the forces between the particles, allowing them to move apart and spread out. The particles themselves remain unchanged, only their arrangement changes.
Examiner tips
- Use the word ‘physical change’ and explain it is a change of state.
- Mention that the chemical composition stays the same and the same particles are present.
- Show that energy is needed to overcome intermolecular forces.
- State that the particles move apart but are unchanged.”]
Mark scheme (4 marks)
- Boiling involves a change of state (from liquid to gas) / the physical form of the substance changes
- The chemical composition of the substance stays the same / the same particles/molecules are present before and after
- Energy is supplied to overcome / break the forces between particles
- The particles move apart / spread out and become a gas, but the particles themselves are unchanged
Key terms in this question
physical change · chemical change
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