Explain why evaporation from the surface of a liquid causes the temperature of the remaining liquid to decrease.

IB DP Physics Standard 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)

Molecules in a liquid possess a range of kinetic energies. Only those at the surface with the highest kinetic energies can overcome the attractive forces and escape as vapour. The escaping molecules therefore carry away more kinetic energy than the average of the liquid. Consequently the average kinetic energy of the remaining molecules falls, which means the temperature of the liquid decreases.

Examiner tips

Use the word ‘evaporation’ and link it to surface molecules with high kinetic energy.
Show the energy transfer: escaping molecules carry away excess energy, lowering the average.
Mention that temperature is proportional to average kinetic energy.
Keep the answer concise – 4 points can be covered in 3–4 sentences.

Common mistakes

Saying the liquid simply ‘loses heat’ without explaining the kinetic energy distribution.
Confusing evaporation with boiling – not all surface molecules evaporate, only the high‑energy ones.
Failing to state that the temperature drop is due to a decrease in average kinetic energy rather than a loss of heat energy.

Mark scheme (4 marks)

Molecules in a liquid have a range / distribution of kinetic energies.
Only molecules with the highest kinetic energies (at the surface) have sufficient energy to overcome intermolecular attractive forces and escape.
The escaping molecules carry away more than the average kinetic energy, so the average kinetic energy of the remaining molecules decreases.
A decrease in the average kinetic energy of the molecules means a decrease in temperature.

Key terms in this question

evaporation · temperature