At any temperature, some of the molecules in a liquid will be moving so fast when they encounter the liquid-gas interface that they keep right on going into the gas phase as vapor.

Most of the molecules of liquid have too little energy, and are pulled back from the interface into the liquid by the attractions of their neighbors. The overall free energy per mole of liquid rises as the temperature increases, and can be thought of as an average escaping tendency of molecules from the liquid.

At the same time, molecules in the vapor above the liquid also have a range of speeds and energies, and the slower-moving among them may be captured when they strike the liquid surface.

The likelihood that this will happen increases with the number of gas molecules hitting the liquid surface per second, which in turn depends on the concentration or partial pressure of vapor molecules above the liquid.

The higher this partial pressure of vapor, the more frequently the molecules will strike the surface of the liquid, and the greater will be the tendency of vapor molecules to mo ve back into the liquid.