When a puddle of water evaporates on a warm day, the ground beneath the puddle becomes cooler. Explain why evaporation causes cooling, using ideas about the kinetic particle model of matter.

Cambridge International IGCSE Physics (0625) — 2.1 Kinetic particle model of matter · 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 the liquid have a range of kinetic energies.
Only the most energetic molecules escape from the surface.
The average kinetic energy of the remaining molecules decreases.
A decrease in average kinetic energy means a decrease in temperature.

Examiner tips

Use the exact wording from the mark scheme – mention ‘range of kinetic energies’, ‘most energetic molecules’, ‘average kinetic energy’ and ‘temperature’.
Show the logical sequence: liquid → escape of high‑energy molecules → lower average energy → cooling.
Keep the answer concise – one sentence per point to fit the 4‑mark structure.

Common mistakes

Failing to mention that only the most energetic molecules evaporate.
Using vague terms like ‘hot’ or ‘cold’ instead of ‘kinetic energy’ and ‘temperature’.
Omitting the link between average kinetic energy and temperature.

Mark scheme (4 marks)

Molecules in the liquid have a range / distribution of kinetic energies
Only the most energetic (higher energy) molecules escape from the surface
The average kinetic energy of the remaining molecules decreases
A decrease in average kinetic energy means a decrease in temperature

Key terms in this question

evaporation