What is saturated vapor pressure?

• A. The equilibrium vapor pressure of a liquid at a given temperature.
• B. The external pressure required for a liquid to boil.
• C. The vapor pressure of a liquid at absolute zero.
• D. The atmospheric pressure at the liquid's surface.

Answer: (A)

Saturated vapor pressure is the pressure of the vapor when a liquid and its vapor are in balance at a specific temperature. In a closed system, molecules constantly evaporate from the surface and some of them condense back into the liquid. When the rates of evaporation and condensation are equal, the vapor pressure stops changing and is called the saturated vapor pressure for that temperature. This pressure represents how much vapor the liquid can exert at that temperature, and it rises as temperature increases because more molecules have enough energy to escape the liquid. Boiling happens when the external pressure equals this saturated vapor pressure; lowering the external pressure below it allows boiling to start. At very low temperatures near absolute zero, the vapor pressure is nearly zero because so few molecules have enough energy to become vapor.