Evaporation, or VAPORISATION, is the process by which a substance changes into the state of vapour. Some solids visibly evaporate—e.g. solid ice or snow gradually disappears though the temperature may be considerably below the freezing-point. The characteristic smell of certain metals, such as copper or iron, is supposed to be due to slow evaporation (see SUBLIMATION). The change of a liquid into the state of vapour may go on at all temperatures; but the rate of change is greater the higher the temperature is, until at last boiling—i.e. free evaporation—occurs (see BOILING). Slow vaporisation at temperatures below the ordinary boiling-point is used on a large scale for commercial purposes, when it is wished to crystallise out a substance which is held in solution. The rate of evaporation depends upon the amount of surface exposed. It depends, too, upon the pressure under which the vapour is developed, and thus evaporation goes on quickly if the vapour is removed as rapidly as it is formed, as is the case when wind blows over the surface of water or snow. The temperature at which boiling occurs also depends upon the pressure. This may readily be shown by boiling water in a glass flask until the steam has driven out most of the air. If the flask be now corked, and the source of heat be withdrawn, the boiling will cease. But if cold water be poured on the flask the boiling will recommence at once. This is due to the fact that the lowering of temperature causes condensation of some of the steam, and this lowers the vapour-pressure so far as to allow of free evaporation at the reduced temperature. At any given temperature evaporation ceases when a certain pressure of the vapour is arrived at, or, more correctly, a state of equilibrium is reached in which condensation balances evaporation. Condensation occurs at all pressures; but, if the proper pressure corresponding to the temperature is not reached, evaporation exceeds condensation; while, if the vapour-pressure is too great, condensation exceeds evaporation. The presence of another gas or vapour, such as ordinary air, has no effect upon the final value of the pressure which is attained; but the time taken to reach this final pressure does depend very greatly upon the presence of another gas. The time becomes greater as the amount of gas present increases. This state of equilibrium is disturbed at once by any variation of vapour-pressure or of temperature. A vapour is said to be saturated when it is in equilibrium in contact with its liquid.
A considerable amount of heat is absorbed in the process of evaporation. The amount depends upon the nature of the liquid, and also upon the temperature of evaporation. Regnault gives for 'total heat' of steam the formula , in which represents the temperature in degrees Centigrade. This means that a pound of saturated water-vapour at . gives out 606.5 units of heat when it condenses to water at ., while a pound at . would give out —i.e. 637 units in changing to water at . The heat given out by a pound of saturated steam at any temperature when it condenses to water at the same temperature is called the latent heat of the steam at that temperature. The term is not a good one. It has reference to the fact that the heat which is required to evaporate a substance without change of temperature disappears so far as ordinary thermometric methods of determining its presence are concerned. If we take into account the variation of the specific heat of steam with temperature, Regnault's measurements of the total heat show that at . the latent heat is 605.5 units, at . it is 536.5 units, and at . it is 464.3 units. It thus decreases very rapidly with rise of temperature. It vanishes at the Critical Temperature (q.v.). The following is the latent heat of vaporisation of a few liquids at a pressure of one atmosphere: water, 536.0; alcohol, 202.4; ether, 90.5; bromine, 45.6. It is evident from these figures that the latent heat of vaporisation of water is relatively large. Were it not for this fact we might have rapid condensation of moisture in the atmosphere in consequence of a very small fall of temperature, and in this way destructive floods might be caused.
Practical use is frequently made of the latent heat of evaporation. Thus, in order to keep water cool it should be stored in vessels of porous earthenware. Some of the water passes through the pores, and evaporates at the outer surface. The latent heat necessary for this is drawn largely from the rest of the water, which is thus kept cool. It is even possible, as was first done by Sir John Leslie, to freeze water by this process. He put some water contained in a shallow earthenware vessel inside the receiver of an air-pump along with a dish of strong sulphuric acid. When the air was exhausted, rapid evaporation resulted; but the vapour was absorbed by the acid as quickly as it was formed, and thus the evaporation was kept up continuously, so that the remaining water was rapidly cooled down to the freezing-point.
The same principle is used in the formation of solid carbonic acid. A jet of liquid acid is allowed to escape from a strong vessel containing it. The outer parts of the jet evaporate, and in consequence the inner parts are solidified. Faraday actually froze mercury in a white-hot vessel by placing it in a little capsule which rested upon a mixture of solid carbonic acid and ether in the spheroidal state.