Evaporation

A liquid is known to increase in temperature when heat is applied. This statement is true up to a certain point called the boiling point of the liquid. When the boiling point is reached, however, adding heat to the liquid no longer raises the temperature. The added heat will then cause a change of state since the liquid will be transformed into a gas or a vapor.

Evaporation also called vaporization is the name given to the process, which occurs when some of the molecules of a liquid tear themselves away from the liquid surface and escape into the air. These molecules form the vapor above the liquid. To tear itself away from the liquid, the molecule, which leaves it, should have a large amount of kinetic energy as the molecular attraction which tends to oppose this escape must be overcome by the molecule. Those molecules which escape must have greater energy than the average kinetic energy of the liquid as a whole.

The kinetic energy of the molecules is in a sense a measure of the temperature of the liquid; and if the molecules with the larger amount of energy escape, the average amount of the kinetic energy of the remaining molecules becomes lower. We, therefore, expect the process of evaporation to lower the temperature of the liquid, and observation shows us that such is really the case.

If little ether is poured on to the hand, a sensation of cold is felt as evaporation takes place. This is because heat is absorbed from the hand to transform the liquid into vapor. The change of state from a liquid to a vapor involves absorption of heat, just as does a change from solid to liquid. Different liquids evaporate at different rates because of differences in their molecular attractions and in their molecular speeds. Mercury, for instance, evaporates very slowly, ether vaporizing" rapidly.

The rate of evaporation also depends on the area of the evaporating surface. That is why water will dry up from a large flat vessel much sooner than it will from a tall and narrow vase.

Evaporation takes place at all temperatures. There is another process, however, which takes place at a particular temperature and at which the process of vaporization is hastened by the constant heat application.

In the process of boiling, heat is constantly added to the liquid. The heat to be added increases the kinetic energy of the molecules, which is the same as saying that the temperature of the liquid starts rising. In the process of evaporation described above the phenomenon is a surface effect. The vapor molecules pass from the surface of the liquid into the air surrounding the liquid. Boiling is a similar process except that when liquid boils evaporation takes place throughout the volume of the liquid, small bubbles of vapor forming within the liquid and additional vapor molecules joining each of the bubbles as it rises to the surface.

In certain cases a solid may change directly into a vapor without undergoing liquefaction. The vapor pressure of a solid at any temperature being greater than one atmosphere, the substance will pass directly from the solid to the vaporous condition. By increasing the pressure, however, the substance can be obtained in a liquid state, provided the change from a liquid to a solid is accompanied by an expansion.