What is defined as the pressure of a vapor in equilibrium with the condensed phase?

The pressure of a vapor that is in equilibrium with its condensed phase—whether solid or liquid—is known as vapor pressure. This concept is fundamental in understanding phase transitions, as it represents the tendency of a substance to evaporate. At a given temperature, the vapor pressure is determined by the balance between the molecules leaving the condensed phase and those returning to it, leading to a stable condition where the rate of evaporation equals the rate of condensation.

Vapor pressure varies with temperature: as temperature increases, more molecules have sufficient energy to escape the condensed phase, resulting in a higher vapor pressure. This property is critical in several phenomena, such as boiling, where the vapor pressure of a liquid equals the external pressure, allowing it to change into vapor.

The other options refer to different concepts. Critical pressure relates to the pressure required to liquefy a gas at its critical temperature. Melting pressure is the pressure at which a solid can melt at a given temperature. Boiling pressure typically refers to the pressure at which a liquid boils under certain conditions, such as atmospheric pressure. Understanding these definitions helps to clarify the unique role of vapor pressure in thermodynamics and phase transitions.