When water boils, it undergoes a physical change rather than a chemical change. Explain the difference between a physical change and a chemical change, and use particle theory to explain what happens to the particles of water when it boils.

WJEC GCSE Chemistry (Wales) — 2.5 The production of important chemicals · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

A physical change involves a change in state but the chemical composition remains the same – no new substance is formed.
A chemical change requires a chemical reaction and produces substances with a different chemical composition to the reactants.
When water boils, the forces between the particles are overcome or broken.
The particles gain energy and move apart, spread out and become a gas.

Examiner tips

  • Use the word "physical" and "chemical" explicitly. Show that boiling is a change of state, not a reaction. Mention particle energy and forces being overcome. Keep the answer concise to fit 4 marks.

Common mistakes

  • Confusing boiling with evaporation. Saying water turns into a new substance. Omitting the role of particle energy or forces.

Mark scheme (4 marks)

  1. A physical change involves a change in state but the chemical composition remains the same / no new substance is formed
  2. A chemical change requires a chemical reaction and produces substances with a different chemical composition to the reactants
  3. When water boils, the forces between the particles are overcome / broken
  4. The particles gain energy and move apart / spread out / move freely / become a gas

Key terms in this question

physical change · chemical change

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