In all the preceding chapters we have focused primarily on thermodynamic systems comprising pure substances. However, in all of nature, mixtures are ubiquitous. In chemical process plants – the ultimate domain of application of the principles of chemical engineering thermodynamics – matter is dominantly processed in the form of mixtures. Process streams are typically comprised of multiple components, very often distributed over multiple phases. Separation or mixing processes necessitate the use of multiple phases in order to preferentially concentrate the desired materials in one of the phases. Reactors very often bring together various reactants that exist in different phases. It follows that during mixing, separation, inter-phase transfer, and reaction processes occurring in chemical plants multi-component gases or liquids undergo composition changes. Thus, in the thermodynamic description of such systems, in addition to pressure and temperature, composition plays a key role.

Further, whenever multiple phases are present in a system, material and energy transfer occurs between till the phases are in equilibrium with each other, i.e., the system tends to a state wherein the all thermal, mechanical and chemical potential (introduced earlier in section 1.4) gradients within and across all phases cease to exist.

The present chapter constitutes a systematic development of the concept of a new class of properties essential to description of real mixtures, as well of the idea of the chemical potential necessary for deriving the criterion of phase and chemical reaction equilibrium. Such properties facilitate the application of the first and second law principles to quantitatively describe changes of internal, energy, enthalpy and entropy of multi-component and multiphase systems.