Air normally contains water vapor (water in the gas phase), and it comes mainly from evaporation. Consider a closed container that is partially filled with water and from which the air has been removed. the fastest moving molecules quickly evaporate in the empty space above the liquid's surface. As they move about, some of these molecules strike the liquid surface and again become part of the liquid phase(this is called condensation). The number of molecules vapor increase until a point is reached when the number of molecules returning to the liquid equals the number leaving in the same time interval. Equilibrium then exits, and the space above the liquid surface is said to be saturated. The pressure of the vapor when it is saturated is called the saturated vapor pressure ( or vapor pressure)