Explain why evaporation causes a liquid to cool, and state two ways in which the rate of evaporation of a liquid can be increased.

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

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

Model answer (4 marks)

Higher‑energy molecules escape from the surface of the liquid. The remaining molecules have a lower average kinetic energy, so the temperature of the liquid falls.

Increasing the temperature increases the rate of evaporation.
Increasing the surface area or creating a draught over the surface increases the rate of evaporation.

Examiner tips

  • Use the exact phrase "higher‑energy molecules" and "lower average kinetic energy" to gain full marks for the explanation. Mention both temperature and surface area/draught as the two ways to increase the rate. Keep the answer concise – 4 marks only.
  • common_mistakes
  • :
  • Saying "evaporation cools because water molecules lose heat" without linking to kinetic energy. Forgetting to state both temperature and surface area/draught as methods to increase evaporation.

Mark scheme (4 marks)

  1. Higher-energy (more energetic / faster-moving) molecules escape from the surface of the liquid
  2. The remaining molecules have a lower average kinetic energy, so the temperature of the liquid falls
  3. Increasing the temperature increases the rate of evaporation
  4. Increasing the surface area OR creating a draught over the surface increases the rate of evaporation

Key terms in this question

evaporation

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