Explain why the temperature of a liquid remains constant during boiling, even though thermal energy is continuously being supplied to it.

Cambridge International IGCSE Physics (0625) — 2.2 Thermal properties and temperature · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

During boiling the supplied energy is used to break the intermolecular forces that hold the liquid molecules together. This energy is the latent heat of vapourisation, not kinetic energy. Because the kinetic energy of the molecules does not increase, the average kinetic energy – and therefore the temperature – remains constant. Thus the temperature stays at the boiling point while the liquid continues to boil.

Examiner tips

  • Use the term ‘latent heat of vapourisation’ to show you know the energy involved.
  • Explain that temperature is linked to average kinetic energy, which is unchanged during phase change.
  • Show the energy goes into overcoming intermolecular forces, not into heating the liquid.

Common mistakes

  • Saying the temperature rises during boiling – it does not.
  • Confusing latent heat with sensible heat – the energy is not used to raise temperature.
  • Using vague terms like ‘heat energy’ without specifying latent heat of vapourisation.

Mark scheme (4 marks)

  1. Energy supplied goes into breaking/overcoming intermolecular forces (rather than increasing kinetic energy)
  2. The kinetic energy of the molecules does not increase (during boiling)
  3. Temperature is a measure of the average kinetic energy of the molecules
  4. The energy supplied during boiling is the latent heat (of vaporisation)

Key terms in this question

temperature

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