Evaporation – IGCSE Chemistry Definition

Evaporation is the escape of particles from the surface of a liquid at temperatures below the boiling point. It is distinct from boiling and is tested on the 0620 syllabus both as a definition and as a particle-theory explanation. The cooling effect of evaporation is a favourite Supplement question.

The 0620 definition

Evaporation is the change of state from liquid to gas that occurs at the surface of a liquid at any temperature below the boiling point.

Particle explanation

In any liquid, particles have a range of kinetic energies. Some particles at the surface happen to have enough energy to overcome the intermolecular forces holding them in the liquid. These particles escape as gas. Because only the most energetic particles leave, the average kinetic energy of the remaining liquid falls — so the liquid cools.

Factors affecting the rate of evaporation

Factor	Effect	Explanation
Temperature	Higher temp = faster	More particles have enough energy to escape
Surface area	Larger area = faster	More particles are exposed at the surface
Air movement (draught)	Faster air = faster	Vapour is carried away, preventing particles returning
Intermolecular forces	Weaker forces = faster	Less energy needed for particles to escape

Evaporation vs boiling

Feature	Evaporation	Boiling
Temperature	Any temperature	At the boiling point only
Location	Surface only	Throughout the liquid
Bubbles	No	Yes
Speed	Slow and gradual	Fast and vigorous
Cooling effect	Yes — removes high-energy particles	No net cooling — external heating maintains temperature

Worked exam question

A student places a drop of ethanol on the back of their hand. The skin feels cold. (a) Name the process occurring as the ethanol disappears. (1) (b) Explain, in terms of particles, why the skin feels cold. (3)

Mark scheme

(a) Evaporation [1]

(b) Particles at the surface of the ethanol with the most kinetic energy escape / change to gas [1]; this lowers the average kinetic energy of the remaining particles [1]; energy is taken from the skin / heat transfers from skin to ethanol, so the skin cools [1]

Common exam mistakes

• Saying “evaporation happens at the boiling point”. Evaporation happens at any temperature — that is its defining difference from boiling.
• Writing “the liquid gets colder” without the particle explanation. You must state that the most energetic particles leave, reducing the average kinetic energy.
• Confusing evaporation with boiling when answering questions about puddles drying or wet clothes drying. These are evaporation — no external heating to the boiling point is involved.