What describes vapor pressure?

Vapor pressure is defined as the pressure exerted by a vapor that is in equilibrium with its liquid phase at a given temperature. This means that in a closed system, as a liquid evaporates, some of its molecules enter the vapor phase, leading to an increase in vapor pressure until a balance is reached between the number of molecules leaving the liquid and those returning from the vapor to the liquid.

At this point of equilibrium, the pressure of the vapor remains constant despite further evaporation or condensation, provided that the temperature remains unchanged. This phenomenon explains why vapor pressure is crucial in understanding phase changes and the behavior of substances in different states. In contrast, the other choices describe related but distinct concepts, such as surface tension, the amount of vapor without reference to equilibrium, or the effects of temperature on gas behavior, which do not specifically capture the essence of what vapor pressure entails.