VAPORIZATION, a general term denoting the change of state of any substance from solid or liquid to vapour. The con verse change from vapour to liquid or solid is most often called condensation, though there are many other special terms employed in particular cases. The general phenomena of change of state as affected by heat have been described in the article HEAT. The most accurate methods of measuring the latent heat of vaporiza tion or condensation are discussed in the article CALORIMETRY, and the application of the laws of thermodynamics to the subject is included in the article THERMODYNAMICS. Important practical applications of the properties of vapours will be found in other articles, such as LIQUEFACTION OF GASES ; DISTILLATION ; STEAM; and STEAM-ENGINE; TURBINE. The chief object of the present article is to discuss the physical explanation of the phenomena, and to show how the various properties of vapours can be repre sented by equations, so that the effects to be expected under any conditions can be calculated.
In effect all gases may be regarded as vapours, since all may be liquefied and solidified under suitable conditions. Conversely we should expect that all vapours would behave as gases under conditions sufficiently far removed from those at which the corresponding liquid can exist. This is found to be the case for all stable vapours, which approximate more and more closely to the gas equation aPV= RT as the temperature is raised or the pressure reduced. In this equation V represents the volume occupied by unit mass at a temperature T on the absolute scale, and pressure P in any convenient units. The constant a is the factor required for reducing PV to heat units, and the value of the constant R in heat-units per degree is 1.985/m, where m is the molecular weight corresponding to the chemical formula for the molecule of the vapour considered. At any temperature below the boiling-point of the liquid, if the volume of the vapour is reduced, the deviations from Boyle's law will seldom exceed I per cent. But a sharp limit is set to the application of the gas equation by the saturation-pressure at which the vapour begins to condense. The pressure then remains constant, as the volume is further reduced at constant temperature, until the whole of the vapour is condensed. By experiments of this kind, Dalton estab lished the law of saturation pressure, that for each vapour there is a unique relation between pressure and temperature defining the state in which alone the liquid and vapour can exist together in equilibrium.
Biot's formula, though apparently the most ungainly and diffi cult to work, possesses some practical interest because it was adopted by Regnault (1847) for representing his observations on the saturation pressures of steam, which were made with a mercury manometer 7o ft. high, and were much the most accurate
obtained for many years. In spite of the five empirical constants in Biot's formula, Regnault found it necessary to use different formulae above and below Ioo° C. At that time the laws of thermodynamics had not been elucidated sufficiently to permit their application to the problem, but some theoretical basis might have been expected in the case of the later formulae.
The law according to which the saturation pressure of any vapour varied with the temperature was discovered by Carnot in 1824 by the direct application of his principle to the case of vaporization, and was first stated in the form :—"The increase of saturation pressure per degree is equal to the latent heat per unit increase of volume in vaporization multiplied by a function of the temperature (Car not's function) which is the same for all substances." The law could not be applied until the form of this function had been determined. Accordingly Carnot utilized the relation in the first instance for calculating values of his function from the very scanty and inaccurate data available for the latent heats of various sub stances. But this failed to give any conclusive result owing to the poverty of the data. (See THERMODYNAMICS.) Clapeyron (1834) first stated the equation in the analytical form in which it is generally known as Clapeyron's equation, in which L is the latent heat of vaporization corresponding to the increase of volume V–v from liquid to vapour, F't is Carnot's function of the temperature and
denotes the increase of the saturation pressure per degree. Ultimately, as the result of Joule's experiments on the mechanical equivalent of heat, denoted by J, and of Regnault's experiments on the gas-scale of tempera ture, denoted by T, it gradually became obvious (see HEAT) that Carnot's function must be very nearly equal to J/T. The absolute scale of temperature T was accordingly defined on Kelvin's sug gestion by the condition T= J/F't, in which it is understood that the appropriate numerical value of J must be employed, depend ing on the units in terms of which heat and work are measured in the equation. Putting a for I/J, Clapeyron's equation may be written in the form, The expression on the left is the ratio of the latent heat L to the equivalent ap(V–v) of the external work of vaporization in heat units. The expression on the right is the most convenient method of stating the rate of increase of saturation pressure with tem perature, since it has the same numerical value, for a given sub stance under given conditions, in all systems of units. To find the expression for p in terms of T, we have merely to integrate equa tion (2) after substituting appropriate expressions for L and V in terms of p and T.