evaporation

evaporation Evaporation is the process by which a liquid is transformed into a gas. The most common example in the natural world is the evaporation of water from the oceans, lakes, and rivers into the atmosphere. The main difference between a liquid and a gas is that in a gas the molecules are free to move anywhere. Each molecule moves until it collides with another molecule, after which it moves off in another direction. In a liquid the forces of attraction between the molecules bind them together. The binding is not as strong as in a solid, such as ice, but it is strong enough to keep almost all the molecules together as a liquid. The molecules do not all have the same energy; some move faster than others.

A few of the molecules in a liquid have sufficient energy to escape from the binding forces that keep them joined to the other liquid molecules. When these molecules leave the liquid surface they have entered the gaseous state, and are then free to move anywhere. On the other hand, some molecules in the gas, while moving about randomly, will collide with a liquid surface. When this happens, they will become bound to the liquid by the attractive forces, and these molecules will have changed from the gaseous to the liquid state. The net effect of these two processes is evaporation if more molecules are leaving the liquid state to become gas than the other way round. If more molecules are leaving the gas and becoming liquid then the process is condensation.

A state of saturation is reached when the molecules are leaving the liquid state at the same rate as molecules are joining the liquid from the gas. Once the gas has become saturated, the only way to increase the number of molecules leaving the liquid is to supply more energy to increase the energy of the molecules. This can be done by heating the liquid, producing thermal energy. (Heat energy which is used to provide energy for evaporation rather than to increase the temperature is called the latent heat of evaporation.) After heating, a new equilibrium level can be reached in which the gas is saturated; that is, the rates at which molecules leave and rejoin the liquid are the same. However, the number of molecules in the gaseous state is now greater than before heating because they have more energy and more molecules have been able to escape the attractive bonds of the liquid. This shows that the state of saturation is temperature-dependent. At higher temperatures gases are capable of supporting more of the molecules from the liquid. This explains why condensation occurs, for example, on windows at night, as the temperature drops. Air, which initially may not have been saturated with water, cools and eventually reaches a temperature at which it is saturated with water. If the air cools below this point it is supersaturated with water, and condensation occurs to restore the balance.

Evaporation and condensation are major factors in the hydrological cycle. Although only about 0.001 per cent of all the water on Earth is in the atmosphere, there is a never-ending cycle of evaporation from the surface, condensation into cloud droplets, and eventually the fall of raindrops and snowflakes returning the water to the surface. Averaged over the entire globe, the annual amount of precipitation would be about a metre deep. By comparison, the total depth of all the atmospheric water vapour at any one time is equivalent to a liquid depth of about 25 mm. To produce the required amount of rain in one year, water must be recycled round the hydrological cycle approximately every nine days.

Charles N. Duncan

Bibliography

Ahrens, C. D. (1994) Meteorology today. West Publishing Co., New York.
McIlveen, J. F. R. (1986) Basic meteorology. Van Nostrand Reinhold, New York.