Explain why a liquid evaporates at temperatures well below its boiling point, and why the rate of evaporation increases when the surface area of the liquid is increased.

IB DP Chemistry Higher 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)

Liquid particles have a range of kinetic energies at any temperature. Some surface particles possess enough energy to overcome intermolecular forces and escape into the gas phase, so evaporation occurs below the boiling point. Increasing the surface area exposes more particles to the surface, allowing more high‑energy particles to leave per unit time. Thus the evaporation rate rises with surface area.

Examiner tips

  • Use the term ‘distribution of kinetic energies’ and ‘surface particles’
  • Explain that evaporation is a surface phenomenon, not bulk
  • Show the link between surface area and number of escaping particles

Common mistakes

  • Confusing evaporation with boiling; stating it only occurs at the boiling point
  • Failing to mention the kinetic energy distribution
  • Not linking surface area to the number of surface particles

Mark scheme (4 marks)

  1. Particles in a liquid have a distribution (range) of kinetic energies at any given temperature.
  2. Some particles at the surface have sufficient kinetic energy to overcome intermolecular forces and escape into the gas phase, even below the boiling point.
  3. Increasing surface area exposes more particles at the surface, so more particles per unit time are able to escape.
  4. Therefore the rate of evaporation increases because more high-energy particles can leave the liquid per unit time.

Key terms in this question

evaporation · surface area · rate of evaporation

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