Thermodynamic properties of pure bodies

Although, in practice, industrial applications mainly concern systems containing a large number of chemical species, it would be simplistic to assume that engineers are not interested in single-component systems. Indeed, the properties of a mixture are calculated from the corresponding properties of the pure compounds. The basic principles and quantities presented in the article "Formalism and Principles of Thermodynamics" in this treatise enable us to describe the thermodynamic properties of pure substances.

The fundamental quantity, from which all the others are deduced, is the chemical potential, which makes it possible to determine in which phase (liquid, vapor or solid) a pure body will be found under given temperature and pressure conditions, and which also makes it possible to predict the existence of the various equilibria between these phases (liquid/solid, liquid/vapor, solid/vapor, solid/solid).

The mathematical formalism used to calculate the thermodynamic properties of pure bodies is general whatever the phase(s) considered: it consists in first considering the properties of the compound assumed to be a perfect gas under the conditions of the experiment, then calculating the deviations from these values using the equations of state that describe the volumetric properties of the phase considered.