Explain why the temperature of water does not increase whilst it is boiling, even though energy is continuously being supplied to it.

AQA GCSE Physics (8463) — 4.3.1 Changes of state and the particle model · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

The energy supplied increases the internal energy of the water.
The added energy is used to break the hydrogen‑bonded network between the water molecules, rather than raising their kinetic energy.
This energy is the latent heat of vapourisation.
Because the phase change occurs at a constant temperature, the temperature of the water remains unchanged while boiling.

Examiner tips

  • Use the term ‘latent heat of vapourisation’ to show understanding of the energy transfer.
  • Explain that the energy goes into breaking intermolecular forces, not increasing kinetic energy.
  • Show the constant temperature during the phase change.
  • Use clear, concise sentences and correct UK spelling.

Common mistakes

  • Confusing the energy supplied with the temperature rise; writing that the temperature increases.
  • Using ‘latent heat of fusion’ instead of ‘latent heat of vapourisation’ for boiling.
  • Omitting the point that the temperature stays constant during the phase change.

Mark scheme (4 marks)

  1. The energy supplied increases the internal energy of the water
  2. The energy is used to break the bonds / overcome the forces between particles (rather than raise temperature)
  3. The energy supplied is the latent heat of vaporisation
  4. The change of state occurs at constant temperature

Related

More Changes of state and the particle model questions

▶ Try answering this question with AI marking (free) →