Water slowly evaporates at room temperature. Gasoline evaporates quite readily without boiling. A liquid that evaporates readily is said to be volatile.
If a quantity of a liquid is contained in a tightly covered container at a certain temperature, molecules will evaporate from the liquid surface into the enclosed space. Since these molecules move about above the liquid and can not escape, some of them will collide with the liquid surface and re-join it (condensation) Fig. 2.15). As a result of the two opposite processes-evaporation and condensation, a state of equilibrium is finally established. This equilibrium is marked by the attainment of a vapor pressure (which is defined as the pressure exerted by the vapours when they are in equilibrium with the liquid phase) It is constant at constant temperature. The vapour pressure is independent of the amount of liquid present. For example the vapour pressure of water at 20°C is 17.5 torr.
The evaporation and the vapour pressure can be explained in terms of kinetic theory. Evaporation occurs when high energy of molecules at the surface of liquid breaks away from their neighbors and escape into the gas phase. During evaporation, the escape of high energy molecules lowers the average kinetic energy of the remaining molecules, consequently the temperature of the remaining liquid falls down. Thus it is said that evaporation is a cooling process.