Describe the differences between boiling and evaporation. In your answer, refer to where each process occurs and the temperatures at which each process can take place.

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

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

Model answer (4 marks)

Evaporation occurs only at the liquid surface, whereas boiling takes place throughout the bulk of the liquid.

Evaporation can happen at any temperature, even below the normal boiling point, because only the highest‑energy molecules leave the surface.

Boiling can only occur at a fixed boiling point; the temperature remains constant while heat supplied is used to change the state.

During evaporation the average kinetic energy of the remaining liquid decreases, but during boiling the temperature stays constant as energy is used for the phase change.

Examiner tips

  • Use the word ‘surface’ for evaporation and ‘bulk’ for boiling; this shows spatial difference. Mention that evaporation can occur below the boiling point, while boiling only at the fixed point. Highlight that evaporation reduces average kinetic energy, whereas boiling keeps temperature constant.
  • common_mistakes
  • :
  • Saying evaporation occurs throughout the liquid. Forgetting that boiling only happens at the boiling point. Not distinguishing the effect on temperature (constant vs. decreasing).

Mark scheme (4 marks)

  1. Evaporation occurs only at the surface of the liquid, whereas boiling occurs throughout the liquid (in the bulk).
  2. Evaporation can occur at any temperature, whereas boiling only occurs at a fixed boiling point.
  3. During evaporation, only the higher-energy molecules escape from the surface, so the average kinetic energy of the remaining molecules decreases.
  4. Boiling occurs at a constant temperature (the boiling point), with energy supplied used to change state rather than increase temperature.

Key terms in this question

evaporation · boiling

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