Vapor pressure is defined as which of the following?

• A. Pressure exerted by air above the liquid
• B. Pressure exerted by vapor when the liquid and vapor are in equilibrium
• C. Pressure required to boil the liquid
• D. Pressure at the container bottom

Answer: (B)

Vapor pressure is the pressure exerted by the vapor in equilibrium with the liquid at a given temperature. When the liquid and its vapor are in balance, molecules continuously evaporate and condense at equal rates, creating a definite pressure in the vapor phase. This pressure grows with temperature because more molecules have enough energy to escape the surface. It isn’t the air pressure above the liquid (that’s atmospheric pressure), nor is it the general external pressure you’d need to cause boiling (boiling happens when external pressure equals the vapor pressure, and that external pressure can vary). It also isn’t the hydrostatic pressure at the bottom of the container.