Explain why the internal energy of a liquid increases as it evaporates, and describe what happens to the arrangement and movement of its particles during this change of state.

Edexcel GCSE Physics (1PH0) — 14.1 Particle model · Explain · 4 marks · View as Markdown

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

A puddle of water left in the sun slowly disappears over several hours as the water evaporates.

Model answer (4 marks)

The internal energy of the water increases because energy is absorbed from the surroundings (the sun) during evaporation. This energy is used to overcome the attractive forces between the liquid molecules, so the potential energy of the particles rises. As a result, the particles move further apart – they are no longer in contact as in the liquid but are widely spaced in the gas phase. The particles then move randomly in all directions with high kinetic energies, unlike the liquid where they flow over one another.

Examiner tips

  • Show the energy absorption step first, then link it to increased potential energy.
  • Explain the change in particle spacing and motion clearly – from close contact to widely spaced and random high‑speed movement.
  • Use the exact terms ‘potential energy’, ‘intermolecular forces’, ‘random motion’ and ‘high kinetic energy’.
  • Mention the source of energy (sun/surroundings) to justify the increase in internal energy.

Common mistakes

  • Failing to state that energy is absorbed from the surroundings, leading to a missing reason for the energy increase.
  • Describing the particles as simply ‘moving faster’ without noting the increase in potential energy and the change in spacing.
  • Using vague terms like ‘more energy’ or ‘more movement’ instead of specific concepts such as ‘potential energy’ and ‘intermolecular forces’.

Mark scheme (4 marks)

  1. The internal energy increases because energy is absorbed (from the surroundings/sun) during evaporation
  2. The potential energy of the particles increases (as intermolecular forces are overcome / particles move further apart)
  3. The particles spread far apart / become widely spaced (from being in contact in the liquid state to being spread apart in the gas state)
  4. Particles move randomly in all directions with high kinetic energies (rather than flowing over each other as in a liquid)

Key terms in this question

internal energy

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