Vapour pressure is defined as which of the following?

Vapour pressure is defined as which of the following?

• A. The pressure exerted by vapour in equilibrium with its liquid at a given temperature.
• B. The atmospheric pressure at which the liquid boils.
• C. The total pressure of all gases in a mixture.
• D. The pressure required to compress a liquid.

Answer: (A)

When a liquid is in a closed space, some molecules escape into the gas phase and, at a given temperature, the rate of evaporation balances the rate of condensation. The pressure exerted by that vapour phase in equilibrium with the liquid is the vapour pressure. It reflects how easily molecules leave the liquid surface and increases with temperature. This definition matches the idea of vapour pressure as the pressure of the vapour in equilibrium with the liquid at that temperature. The other statements describe related but different ideas: the external pressure needed to boil is the boiling point condition, not the inherent vapour pressure; the total pressure of a gas mixture is the sum of partial pressures and not specifically the vapour pressure of the liquid; and the pressure needed to compress a liquid has nothing to do with vapour pressure, since liquids are largely incompressible.

When a liquid is in a closed space, some molecules escape into the gas phase and, at a given temperature, the rate of evaporation balances the rate of condensation. The pressure exerted by that vapour phase in equilibrium with the liquid is the vapour pressure. It reflects how easily molecules leave the liquid surface and increases with temperature.

This definition matches the idea of vapour pressure as the pressure of the vapour in equilibrium with the liquid at that temperature. The other statements describe related but different ideas: the external pressure needed to boil is the boiling point condition, not the inherent vapour pressure; the total pressure of a gas mixture is the sum of partial pressures and not specifically the vapour pressure of the liquid; and the pressure needed to compress a liquid has nothing to do with vapour pressure, since liquids are largely incompressible.