Explain why the temperature of a pure substance does not change during a change of state, even though thermal energy is continuously supplied to it.

IB DP Physics Higher Level (2023 syllabus) — B.1 Thermal energy transfers · 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 a change of state the supplied energy is used to overcome the intermolecular (potential) forces between molecules. Consequently the average kinetic energy of the molecules does not increase. Temperature is a measure of the average random translational kinetic energy, so if the kinetic energy remains constant the temperature remains constant even though energy continues to be supplied.

Examiner tips

  • State that energy is used to break bonds, not to increase kinetic energy
  • Explain the link between kinetic energy and temperature
  • Use the phrase "average kinetic energy"
  • Show the logical sequence: energy input → bond breaking → constant kinetic energy → constant temperature

Common mistakes

  • Confusing temperature with total energy or enthalpy
  • Saying the temperature rises during the phase change
  • Using vague terms like "heat" instead of "thermal energy"

Mark scheme (4 marks)

  1. During a change of state, the supplied energy is used to overcome the intermolecular (potential) forces/bonds between molecules.
  2. The average kinetic energy of the molecules therefore does not increase during the change of state.
  3. Temperature is a measure of the average (random translational) kinetic energy of the molecules.
  4. Since the average kinetic energy remains constant, the temperature remains constant (despite continued energy input).

Key terms in this question

change of state · temperature

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