Water exists as a solid, liquid or gas depending on temperature. Using the particle model, explain what happens to the particles in liquid water as it is heated to its boiling point and then changes into a gas.

Eduqas GCSE Chemistry — 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)

The particles in liquid water gain kinetic energy as the temperature rises.
At the boiling point the kinetic energy is sufficient to overcome the intermolecular forces between the particles, so the forces are broken.
This is a physical change – the chemical composition of the water (H₂O) remains unchanged.
In the gas state the particles are far apart, move freely and possess the greatest kinetic energy.

Examiner tips

  • Use the word ‘kinetic energy’ and ‘intermolecular forces’ – these are key terms. Show the sequence: heating → energy gain → forces broken → gas. Mention that the change is physical, not chemical.
  • Use the particle model language: ‘particles move freely’, ‘most spread apart’ – this demonstrates understanding of the model.

Common mistakes

  • Forgetting to mention that the chemical composition does not change. Using vague terms like ‘more energy’ without specifying kinetic energy. Confusing the boiling point with the melting point or not describing the particle movement in the gas state.

Mark scheme (4 marks)

  1. Particles in the liquid gain (kinetic) energy as they are heated
  2. At the boiling point, the forces between the particles are overcome / broken
  3. This is a physical change — the chemical composition of the water remains the same
  4. In the gas state, particles are the most spread apart / move freely and have the most energy

Key terms in this question

particle model · boiling point

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