Evaporation (the transition of liquid molecules into the)

EVAPORATION, the transition of liquid molecules into the surrounding gaseous space, wherein the transition in a confined space occurs rapidly at first, then more slowly, until finally the entire space is filled with the maximum amount of liquid vapor. The mechanism of evaporation can be conceived in the following manner: liquid particles perform complex motions while being held close to each other by forces of mutual attraction. At any given moment, the particles possess certain velocities, and if a particle's velocity is directed outward and is sufficiently large, the particle escapes the sphere of molecular action and passes into the surrounding space as a gas molecule. During the transition into the vapor state, it frequently happens that liquid particles, consisting of many separate molecules, disintegrate into individual molecules existing in the vapor state, which can occur thanks to the significant velocities of the individual parts of a given molecule leading to its disintegration. The mechanism of evaporation indicated above makes it possible to explain a whole range of phenomena observed during evaporation. First of all, one should expect a sharp influence of temperature on evaporation, since with temperature the velocity of particle motion increases, and the number of particles that have a velocity directed outward and exceeding a certain definite magnitude becomes greater. Therefore, evaporation increases with rising temperature. The transition of molecules into the surrounding limited medium will practically cease when the number of molecules emerging from the liquid is equal to the number of molecules passing into the liquid from the confined space situated above the liquid. Since the number of molecules in the vapor state must depend on temperature and there is a constant transition of molecules from the liquid into the vapor and vice versa, phenomena of mobile equilibrium, changing with temperature, take place here. If a movement of gas is produced along the surface of the liquid, carrying away the molecules that have passed into the gaseous layer, then evaporation is facilitated; moreover, the faster the removal of molecules and the fewer of them there are above the liquid layer, the faster the evaporation of molecules moving away from the surface occurs. During evaporation, work is expended to overcome the forces holding the molecules at the surface. These forces are forces of molecular attraction, and the kinetic energy of the liquid particles is spent on overcoming this work. Thus, the liquid particles passing into vapor must possess less kinetic energy than they possessed in the liquid. Such a loss of kinetic energy manifests itself in the fact that the liquid and vapor gradually begin to cool if there is no influx of heat from the outside. If evaporation occurs continuously and the departing molecules are carried away by the air and do not prevent the remaining molecules from evaporating, then in order to maintain the temperature of the liquid at a certain level, it is necessary to constantly supply it with energy that will compensate for the work expended on evaporation. Per unit weight of evaporated liquid, a certain constant amount of heat must always be supplied to it at a given temperature, which compensates for the loss of energy spent on evaporation. An increase in temperature will thus not occur, and the temperature of the liquid will remain constant. The amount of heat necessary for the evaporation of a gram of liquid and going toward overcoming internal molecular forces without raising the temperature is called the latent heat of evaporation. Obviously, the latent heat of evaporation must depend on the temperature at which evaporation takes place.