How is vapor pressure defined?

Vapor pressure is defined as the pressure exerted by a vapor in equilibrium with its liquid. This balance indicates that at a given temperature, the rate at which molecules escape from the liquid to the vapor phase equals the rate at which they return to the liquid phase. This dynamic equilibrium describes a situation where, despite molecules continuously transitioning between the liquid and vapor states, the overall pressure exerted by the vapor remains constant.

Understanding vapor pressure in this context is crucial because it emphasizes the relationship between a liquid and its corresponding vapor at a specific temperature. When measuring vapor pressure, it is important to note that the system must be closed to prevent any gas from diffusing away, ensuring the vapor can achieve equilibrium with the liquid. This definition distinguishes vapor pressure from other types of pressures, such as the total pressure of a gas mixture or pressures associated with gases at high temperatures, which do not involve direct equilibrium considerations between a liquid and its vapor phase.