Condensation – IGCSE Chemistry Definition

Condensation is the reverse of boiling: a change of state from gas to liquid. The 0620 syllabus expects you to name the process, explain it using particle theory, and recognise it on cooling curves. It also appears in practical contexts such as distillation, where vapour condenses in the condenser to form a liquid distillate.

The 0620 definition

Condensation is the change of state from gas to liquid. It occurs when gas particles lose energy, slow down, and intermolecular forces bring them close enough together to form a liquid.

Particle explanation

Gas particles are widely spaced and move rapidly with high kinetic energy. When a gas is cooled:

1. Particles lose kinetic energy and slow down
2. Intermolecular forces of attraction become strong enough to hold particles closer together
3. Particles form a liquid — still able to move past each other but much closer together than in a gas

No chemical bonds are formed or broken — only intermolecular forces change.

Condensation on cooling curves

On a cooling curve (temperature vs time for a substance cooling from gas to solid), condensation appears as a horizontal plateau at the boiling point. During this plateau:

• Temperature remains constant
• The substance is changing from gas to liquid
• Energy is being released to the surroundings (exothermic)
• Both gas and liquid are present

Condensation in distillation

In simple distillation, a mixture is heated until one component boils. The vapour passes into a condenser — a tube surrounded by cold water. The vapour cools, condenses back to a liquid, and is collected as the distillate. Condensation is the step that recovers a pure liquid from the vapour.

In fractional distillation of crude oil, different fractions condense at different heights in the column because they have different boiling points.

Worked exam question

Water vapour from a boiling kettle meets a cold window and water droplets form. (a) Name the change of state taking place. (1) (b) Explain, in terms of particles and energy, why water droplets form on the cold window. (3)

Mark scheme

(a) Condensation [1]

(b) Water vapour particles lose kinetic energy when they hit the cold surface / are cooled [1]; particles slow down [1]; intermolecular forces (of attraction) pull particles close together to form liquid water [1]

Common exam mistakes

• Confusing the everyday meaning of “condensation” (water appearing on a cold surface) with the scientific definition (change of state from gas to liquid). The water droplets form because water vapour in the air condenses — always use the particle explanation.
• Writing that “bonds form” without specifying intermolecular forces. Covalent bonds within water molecules already exist and do not change during condensation.
• Forgetting that condensation is exothermic. Energy is released when intermolecular forces form, not absorbed.