What is vapor pressure?

• A. The atmospheric pressure at which a liquid boils
• B. The pressure the vapor exerts on its container when the vapor and liquid are in equilibrium
• C. The partial pressure of dissolved gases
• D. The pressure needed to start condensation

Answer: (B)

Vapor pressure is the pressure exerted by the vapor of a liquid in equilibrium with the liquid itself at a given temperature. In a closed container, some molecules escape into the gas phase, and at equilibrium the rate of evaporation equals the rate of condensation, producing a steady pressure in the headspace—the vapor pressure. This value is specific to the liquid and its temperature and tends to increase as temperature goes up because more molecules have enough energy to escape.

The other descriptions mix up different ideas: boiling pressure refers to the ambient pressure needed for a liquid to boil, which is not the vapor pressure itself; dissolved gases relate to gases dissolved in a liquid (not the vapor above it); and condensation pressure isn’t a standard defined threshold in the way vapor pressure is.