Explain why evaporation of a liquid causes the remaining liquid to cool, using ideas about particles and energy.

IB DP Chemistry Standard Level (2023 syllabus) — S1.1 Introduction to the particulate nature 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)

Particles in a liquid possess a range of kinetic energies. Only those with the highest energies can overcome the intermolecular forces at the surface and escape as vapour. When these high‑energy particles leave, the average kinetic energy of the particles that remain in the liquid decreases. Because temperature is proportional to the average kinetic energy of the particles, the temperature of the remaining liquid falls.

Examiner tips

  • Use the word ‘evaporation’ and link it to ‘surface particles’
  • Show the chain: high‑energy particles escape → average kinetic energy ↓ → temperature ↓
  • Mention ‘intermolecular forces’ to demonstrate understanding of escape energy

Common mistakes

  • Failing to mention that only the highest‑energy particles escape
  • Confusing evaporation with boiling or not linking kinetic energy to temperature
  • Using vague terms like ‘heat loss’ instead of kinetic energy changes

Mark scheme (4 marks)

  1. Particles in a liquid have a range (distribution) of kinetic energies.
  2. Only the particles with the highest kinetic energies have sufficient energy to overcome intermolecular forces and escape from the surface.
  3. When these high-energy particles leave, the average kinetic energy of the remaining particles decreases.
  4. Since temperature is proportional to average kinetic energy of the particles, the temperature of the remaining liquid falls.

Key terms in this question

evaporation

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