What is vapor pressure?

• A. The pressure at which vapor and liquid are in equilibrium at a given temperature
• B. The atmospheric pressure
• C. The pressure of gas in a closed container
• D. The partial pressure of non-condensable gas

Answer: (A)

Vapor pressure is the pressure exerted by the vapor that forms above a liquid when the two phases are in balance at a specific temperature. In a closed system, molecules continuously evaporate from the liquid surface and some return as they condense; at a given temperature, the rate of evaporation equals the rate of condensation, and the vapor above the liquid settles at a characteristic pressure—the vapor pressure. This pressure rises with temperature because more molecules have enough energy to escape into the vapor phase. Boiling occurs when this vapor pressure matches the surrounding atmospheric pressure.

The other descriptions don’t capture this specific concept: atmospheric pressure is the external pressure on the system, not the vapor pressure of the liquid; the pressure of gas in a closed container could include many gases and isn’t necessarily the vapor pressure; and the partial pressure of a non-condensable gas refers to its share of the total pressure, not the pressure of the liquid’s vapor in equilibrium.