EVAPORATION (Lat. eraporatio, from In from e, away + rapo rare, to emit vapor. from vapor, vapor). The change of state from solid or liquid to gaseous. All substances emit vapors, and those which at ordinary temperatures evaporate rapidly are said to be volatile. If the vessel in which a liquid is allowed to evaporate be open, the vapor will diffuse through the air, and evaporation will go on until there is no more liquid left. But if a liquid is placed in an air-tight vessel, evapora tion goes on until the vapor escaping from the liquid has attained a certain definite pressure within the vessel, and then a dynamic equilib rium is established; that is to say, evaporation will not really cease, but for every amount of liquid evaporated an equal amount of vapor will be condensed, and, as a result, the amount of vapor, and hence the pressure, will remain constant. This constant pressure, measuring the vapor-tension of the liquid, depends entirely on two factors—the nature of the liquid and the temperature at which it is kept. At the saint. temperature, different liquids have different vapor-tensions, and when the temperature is raised, the vapor-tension of each is increased by an amount depending, again, upon the nature of the liquid. When the vapor of a substance has
attained its full pressure, it is said to be sat orated. The full vapor-tension corresponding to a certain temperature is not attained instantane onsv, since evaporation requires time. The rate of evaporation depends on a large number of circumstances, sonic of which are more or less accidental, i.e. have nothing to do with the na ture of the liquid experimented upon. Thus the size, shape, and material of the vessel in which evaporation is allowed to take place have a considerable influence on the rate. Other factors are the temperature of the liquid itself, the density of the atmosphere above its surface, and the magnitude of the surface exposed, shallow vessels being employed when rapid and copious evaporation is required. In case the liquid is a mixture or a solution, the rate of evaporation de pends greatly upon its composition and upon the nature of the several constituents.