The thermodynamic modeling of liquids and liquid mixtures is important to both the chemist and the chemical engineer. The chemist is interested in the physical forces that influence the behavior of liquid mixtures. The chemical engineer is interested in modeling the behavior of liquids and liquid mixtures for use in industrial applications, for example to predict the properties of multicomponent mixtures in the petrochemical, pharmaceutical, and refrigeration and air-conditioning industries. Testing the models against experimentally measured properties makes it possible to determine whether the ideas on which the models are based are correct.