Evaporation

In the preceding chapter it was shown how the discovery by Robert Brown in 1827 of the continuous vibration of very small particles, now known as Brownian movement (or motion), led to the kinetic theory of gases. This is central to the process of evaporation. The molecules of liquid water are much closer together than those of a gas and are separated from each other by just slightly more than the diameter of one molecule. In such close proximity, the atomic particles strongly attract each other by Van der Vaals forces (electrical attractions between molecules), but this force reduces rapidly as their separation is increased. In water vapour, the spacing between molecules is ten diameters or more, depending on their concentration (or vapour pressure, VP – that fraction of the total pressure due to the water vapour alone – measured in any of the usual units of pressure, such as millibars or hectopascals) and the attractive force is then extremely small. To produce water vapour from liquid water, the distance between the molecules has to be increased, and to achieve this, work has to be done against the binding Van der Vaals attraction.

During evaporation (at any temperature) water molecules ‘boil off’, due to Brownian movement, at a rate proportional to the absolute temperature. Some, however, by chance, due to their random motion in the air, find their way back to the water surface, the numbers doing so depending on their concentration in the air which is directly related to the VP.