Describe what happens to the internal energy of a block of ice as it is heated from well below 0 °C until it has completely melted, and explain why the temperature does not change while the ice is melting.

AQA GCSE Physics (8463) — 4.3.2 Internal energy and energy transfers · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The internal energy of the ice rises as heat is supplied, because the energy stored in the particles increases.

Below 0 °C the added energy raises the temperature – the kinetic energy of the water molecules increases.

When the ice reaches 0 °C the supplied energy is used as latent heat of fusion; it breaks the hydrogen‑bond network between the molecules rather than increasing their kinetic energy.

Because the kinetic energy does not change during the phase change, the temperature remains constant until all the ice has melted.

Examiner tips

  • Use the term ‘latent heat of fusion’ when explaining the energy used during melting.
  • Show the temperature rise before 0 °C and the constant temperature during the phase change.
  • Mention that the internal energy increases throughout, but the temperature only changes when the phase is unchanged.
  • Use the phrase ‘break the forces between particles’ to match the mark scheme.

Common mistakes

  • Saying the temperature rises continuously through the melting point.
  • Confusing latent heat with specific heat capacity.
  • Using ‘heat of fusion’ without explaining it is latent heat.

Mark scheme (4 marks)

  1. Heating the ice increases its internal energy / the energy stored by the particles increases
  2. Below melting point, the increased internal energy raises the temperature because the kinetic energy of the particles increases
  3. During melting, the energy supplied (latent heat) is used to break / overcome the forces between particles rather than raising the temperature
  4. The kinetic energy of the particles does not change during melting, so the temperature remains constant

Key terms in this question

internal energy · melting

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