Describe what happens to the internal energy of a substance as it melts, and explain why its temperature does not change during melting.

AQA GCSE Physics (8463) — 4.3.1 Changes of state and the particle model · Describe and 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 wax until it completely melts. The student notices that the temperature stays the same while the wax is melting, even though energy is being continuously supplied.

Model answer (4 marks)

The internal energy of the wax rises as it melts.

During melting the supplied energy is used to increase the potential energy of the wax molecules – the energy required to separate them from their fixed positions in the solid.

Temperature depends on the average kinetic energy of the molecules.

Because the kinetic energy does not increase, the temperature remains constant while the wax is melting.

Examiner tips

  • Use the word "increases" for internal energy and "potential energy" for the energy used to break bonds.
  • Explain that temperature is linked to kinetic energy, not potential energy.
  • Show the logical link: energy supplied → potential energy ↑ → kinetic energy unchanged → temperature constant.
  • Keep the answer concise – 4 marks can be earned with two short sentences.

Mark scheme (4 marks)

  1. The internal energy of the substance increases during melting.
  2. The energy supplied increases the potential energy of the particles (not the kinetic energy).
  3. Temperature is related to the average kinetic energy of the particles.
  4. Since the kinetic energy of the particles does not increase, the temperature does not change / remains constant.

Key terms in this question

internal energy · melting

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