14. Binary Systems

The average chemical potential of a mixture depends not only upon the composition but also upon the temperature (and pressure). These dependencies and the fact that the phase with the lowest chemical potential at a given temperature (or pressure) will be stable can be used to construct the phase diagrams of different mixtures. First, we will discuss the temperature–composition diagrams of two liquid phases. With the help of these diagrams, we can judge under which conditions the two liquids are mutually miscible and under which they are not; the diagrams are therefore also called miscibility diagrams. Liquid–solid phase diagrams are used to identify the regions of temperature and composition at which solids and liquids exist in a two-component system. Such diagrams are of great commercial and industrial relevance; they play an important role in metallurgy but also in the manufacture of ceramics and semiconductors. In the last section, the phase diagrams of binary mixtures of two volatile components are discussed. This kind of diagram is important for understanding distillation, one of the most significant processes used in chemical laboratories and industry for separating liquid mixtures. It has been in use since ancient times to extract essential oils such as attar of roses. An important industrial application is distilling of petroleum in oil refineries that produce the heavy and light gasoline used to fuel engines.