The vapour pressure of a given volume of liquid increase when the

Analyze the question: It asks what makes the vapour pressure of a liquid increase.

Concept: Vapour pressure is the pressure exerted by the vapour in equilibrium with its liquid at a given temperature. It depends mainly on:

• the nature of the liquid (the strength of its intermolecular forces)
• the temperature

It does not depend on the volume or amount of liquid.

Step-by-step evaluation:

• A. Liquid becomes saturated with a salt: Dissolving a non-volatile solute lowers the vapour pressure (Raoult's law). So this does not increase it. ✗
• B. Volume of the liquid increases: Vapour pressure is independent of the amount of liquid. A cup of water and a lake have the same vapour pressure at the same temperature. ✗
• C. Temperature of the liquid falls: Lower temperature means fewer molecules have enough kinetic energy to escape, so vapour pressure decreases. ✗
• D. Temperature of the liquid rises: Higher temperature gives molecules more kinetic energy, so more of them escape into the vapour phase. The vapour pressure increases. This is why a liquid boils when its vapour pressure equals the external (atmospheric) pressure. ✓

Answer: D, since the temperature of the liquid rises.

Exam tip: Remember "Vapour pressure goes up with temperature, down with dissolved solute, and doesn't care about volume." In WASSCE, JAMB and NECO, this idea often appears in questions on boiling point and Raoult's law.