Saturated Vapor Pressure

Air normally contains gaseous water, or water vapor, whose main source is evaporation. The process of evaporation can be explained on the basis of kinetic theory. The molecules in a liquid move past one another at different speeds. There are strong attractive forces between these molecules, keeping them together in the liquid phase. A molecule near the surface of the liquid may, because of its speed, leave the liquid momentarily. The combined attractive force of the other molecules can pull the escaping molecule back to the liquid surface if the velocity of this molecule is not too great. However, a molecule with a sufficiently high velocity (and therefore sufficient kinetic energy) will escape the liquid entirely into the gas phase.

Some proportion of liquid molecules have kinetic energy above a particular value that enables them to escape into the gas phase. This proportion increases with temperature.

In a closed glass vessel filled partly with water (or any other liquid) and from which the air has been removed, the fastest moving molecules evaporate into the space above. As they move around, some of these molecules strike the liquid surface and again become part of the liquid phase; this is one form of condensation.

From the initial state of no water vapor in the vessel, the number of molecules in the vapor (gas) phase increases for a period of time, until the number returning to the liquid equals the number leaving in the same time interval. At this state of equilibrium, the vapor is said to be saturated. The pressure of the vapor when it is saturated above the liquid surface is called the saturated vapor pressure.

The value of the saturated vapor pressure does not depend on the volume of the container. If the volume above the liquid was suddenly reduced, the density of molecules in the vapor phase would increase for a short time. More molecules would then return to the liquid phase until equilibrium is reached again. This would result in the same value of saturated vapor pressure as before.

The saturated vapor pressure of any substance depends on its temperature. At higher temperatures, more molecules have sufficient kinetic energy to break from the liquid surface into the vapor phase. Under these conditions, equilibrium is reached at a higher pressure. This relationship between temperature and saturated vapor pressure for water is illustrated in the table below.

Saturated Vapor Pressure of Water at Different Temperatures