The New Student's Reference Work/Evaporation
Evap′oration is the process by which a liquid body changes into a gaseous body. Whenever a vessel is only partly filled with a liquid substance, it is found that the remainder of this vessel always contains some of this substance in a gaseous form. This gas is called vapor. If the vessel be closed, it is found that, at any one temperature, this process of evaporation will go on until the vapor exerts a certain definite pressure—known as the vapor-tension of that substance at that temperature. What is very curious is that this vapor-tension is independent of any other gases that may be present over the liquid. It is not to be supposed that evaporation ceases when the vapor pressure reaches a maximum; but rather that, at this point, as many particles are leaving the vapor to enter the liquid as are leaving the liquid to enter the vapor. The rate at which evaporation proceeds depends very greatly upon the amount of vapor already present over the liquid. Accordingly, anything which diminishes the amount of vapor already over the liquid will increase the amount of evaporation. It is in this manner that a spoonful of tea may be cooled by blowing over it. For by blowing away the vapor we increase the evaporation. Since the average kinetic energy of a particle of vapor is very much greater than the average kinetic energy of a particle of liquid, it is clear that evaporation must rob the liquid of energy and thus cool it. If a watch-glass be placed on a cork with sulphuric ether on the glass and water underneath the glass, as shown in the figure, the water may be frozen almost immediately by simply blowing a steady stream of air over the surface of the ether. The vapor-tension of ether is high even at ordinary temperatures, so that evaporation is rapid. Since the latent heat of the vapor is derived from the heat in the ether, glass and water, the water is quickly frozen.
Another method of increasing evaporation is to increase the surface of the liquid. A pint of water in a large flat pan will evaporate much more rapidly than the same amount of water in an ordinary tin cup. Still another method of hastening evaporation is to place the liquid under the receiver of an air-pump and pump the vapor off. Strong sulphuric acid, which greedily absorbs water-vapor, will act in the same way. By combining these two methods (i. e., placing both water and suphuric acid under the receiver of an air-pump) Leslie succeeded in freezing the water by evaporating it at the expense of its own heat.
Sometimes a solid evaporates directly without passing through the liquid state. Such a process is called sublimation. This is easily observed in the case of camphor and iodine. When a liquid is heated to the point where evaporation is no longer confined to the surface of the liquid but begins in the interior of the liquid, we call the process ebullition, or boiling. The temperature at which this occurs is called the boiling point. Evidently the boiling point may also be defined as that temperature at which the vapor-tension of a liquid is equal to the pressure of the atmosphere.