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

IB DP Physics Higher 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 have a distribution of kinetic energies (Maxwell–Boltzmann). Only the most energetic molecules near the surface can overcome intermolecular forces and escape as vapour. These escaping molecules carry away more than the average kinetic energy, so the mean kinetic energy of the remaining molecules falls. Because temperature is proportional to the mean kinetic energy, the temperature of the liquid decreases.

Examiner tips

Use the phrase "distribution of kinetic energies" to show knowledge of Maxwell–Boltzmann.
Explain that only the fastest molecules escape, linking to surface molecules.
Show that escaping molecules remove more than average energy, causing a drop in mean energy.
State that temperature is proportional to mean kinetic energy to justify the final conclusion.

Common mistakes

Confusing evaporation with boiling (ignoring surface molecules).
Failing to mention that only the fastest molecules escape.
Not linking the loss of kinetic energy to a decrease in temperature.

Mark scheme (4 marks)

Molecules in a liquid have a distribution of kinetic energies / speeds (Maxwell–Boltzmann distribution).
Only the most energetic / fastest-moving molecules near 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 mean kinetic energy of the remaining molecules decreases.
Since temperature is proportional to the mean kinetic energy of the molecules, the temperature of the remaining liquid falls.

Key terms in this question

evaporation · temperature