Boiling

BOILING, a process in which the molecules of a liquid at a certain temperature pass into a vaporous state, with the formation of vapor occurring not only from the surface, as in quiet evaporation (see), but also within the liquid in the form of bubbles, mainly near the walls of the vessel where there are individual points of overheating. Boiling is observed when a liquid is heated to a temperature at which the pressure of its vapor begins to exceed the external pressure. As the external pressure increases, the boiling temperature of the liquid rises; with a decrease in pressure, the boiling temperature also decreases. Thus, if one rapidly removes the vapors of boiling water from a vessel from which air has been removed by pumping, it is possible, with a sufficient speed of vapor removal, to lower the boiling temperature so much that boiling will occur in a completely cold and even freezing liquid. The lowest temperature at which boiling is observed at a given external pressure is called the boiling point, and the pressure corresponding to it is noted in parentheses next to the temperature. The boiling point of a liquid at atmospheric pressure is one of its characteristic physical constants. During boiling, as in the evaporation of a liquid, heat is absorbed (latent heat of vaporization), which is used to move molecules possessing sufficient kinetic energy out of the sphere of action of other molecules. Such molecules are ejected into the space above the liquid, and the work done to overcome the cohesive forces holding the liquid molecules near the surface of the liquid body is the work corresponding to the latent heat of vaporization or boiling. If a liquid is heated in a sealed vessel and its boiling is observed at various pressures of its vapors, it turns out that the latent heat will regularly change, becoming smaller and smaller as the temperature rises. A moment may finally arrive when the latent heat will be equal to 0, when, consequently, no work will be required to convert the given liquid into a gaseous state. Mendeleev called this temperature the absolute boiling point; at present, it bears the name critical temperature, being one of the characteristic properties of a liquid. Boiling usually occurs in such a way that gases are first released from the liquid, which carry with them the evaporating liquid contained in the gas bubbles. The gas released during boiling is removed first mainly from the walls of the vessel, and as boiling proceeds further and further, the processes of gas bubble formation become more and more difficult. In the complete absence of gases, overheating of the liquid occurs, and it boils in separate jolts. To maintain uniform boiling, bodies containing gases (air) are introduced into the liquid, for example, talc, glass capillaries sealed at one end, pieces of platinum, clay shards, thread, etc.