Explain what happens to the internal energy and the temperature of a substance as it melts, and explain why the temperature does not change during melting even though energy is being supplied.

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 student heats a block of ice and notices that, once the ice begins to melt, the temperature of the mixture stays at 0 °C until all the ice has melted, even though the heater is still supplying energy.

Model answer (4 marks)

During melting, the energy supplied is latent heat (the specific latent heat of fusion). This energy increases the potential energy of the particles – the intermolecular forces are overcome and the particles move further apart. The kinetic energy of the particles does not increase during melting. Temperature is a measure of the average kinetic energy of the particles, so because the kinetic energy remains constant, the temperature stays at 0 °C until all the ice has melted.

Examiner tips

  • Use the term ‘latent heat of fusion’ and mention potential energy increase.
  • Explain that temperature is linked to kinetic energy, not potential energy.
  • Show the chain: energy supplied → potential energy ↑, kinetic energy unchanged → temperature constant.
  • Keep the answer concise and use the exact wording from the mark scheme.

Common mistakes

  • Confusing latent heat with sensible heat or saying the temperature rises during melting.
  • Saying the energy increases kinetic energy instead of potential energy.
  • Using vague phrases like ‘energy is used to melt’ without linking to latent heat and temperature behaviour.

Mark scheme (4 marks)

  1. During melting, energy is supplied as latent heat (specific latent heat of fusion) to change the state of the substance.
  2. The energy supplied increases the potential energy of the particles (as intermolecular forces are overcome / particles move further apart).
  3. The kinetic energy of the particles does not increase during melting.
  4. Temperature is a measure of the average kinetic energy of the particles, so because the kinetic energy does not change, the temperature remains constant.

Key terms in this question

internal energy · temperature

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