Evaporation from the surface of a liquid causes the liquid to cool. Explain why the liquid cools as evaporation takes place.

Pearson Edexcel International GCSE Physics (4PH1) — 5.2 Change of state · Explain · 4 marks · View as Markdown

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

Nurses sometimes wipe a patient's skin with a liquid alcohol solution before giving an injection. The patient feels a cooling sensation on their skin as the alcohol evaporates.

Model answer (4 marks)

Only molecules with the highest kinetic energy escape from the surface during evaporation. The remaining molecules therefore have a lower average kinetic energy. Temperature is a measure of the average kinetic energy of the particles, so a lower average kinetic energy means a lower temperature. Evaporation occurs at the surface of the liquid and can happen at any temperature (not just the boiling point).

Examiner tips

Use the exact wording from the mark scheme – e.g. ‘highest kinetic energy’ and ‘average kinetic energy’.
Explain the link between kinetic energy and temperature clearly.
Mention that evaporation can occur at any temperature, not just the boiling point.
Keep the answer concise – 4 short sentences will earn full marks.

Common mistakes

Confusing evaporation with boiling – forgetting to state it can happen at any temperature.
Using vague terms like ‘heat loss’ instead of ‘average kinetic energy’.
Omitting the point that only the highest‑energy molecules escape.

Mark scheme (4 marks)

Only molecules with the highest kinetic energy escape from the surface during evaporation
The remaining molecules have a lower average kinetic energy
Temperature is a measure of the average kinetic energy of the particles, so a lower average kinetic energy means a lower temperature
Evaporation occurs at the surface of the liquid and can happen at any temperature (not just the boiling point)

Key terms in this question

evaporation