Chapter 2: Properties of Pure Substances

In Secs. 1.3 and 1.4 we have been introduced to some assumptions used in classical thermodynamics: continuum, homogeneity, equilibrium. Another assumption is that we deal with pure substances; these substances have a fixed chemical composition throughout. This is very important when analyzing thermodynamic processes involving phase changes. A pure substance does not have to be a single chemical element; therefore water, oxygen, nitrogen, carbon dioxide are valid examples. Homogeneous mixtures of gases, such as air, can also be considered pure substances as long as there is no change of phase.