The study of phase equilibrium affords methods by which the excess chemical potentials of the components in a solution may be determined. It is concerned with the relations that exist between the intensive variables—temperature, pressure, and composition of the phases—when two or more phases are in equilibrium. In this respect, such studies also yield information on the colligative properties of a solution. For single-component systems without restrictions, the temperature and pressure are the only variables of interest. When restrictions are applied, an additional number of temperature or pressure variables must be introduced. In experimental studies, conditions are set so that any given system is univariant and it is possible to determine the dependence of one variable, a temperature or pressure, on one other variable, again a temperature or a pressure. Composition variables must be used for multicomponent systems, in addition to the temperature and pressure, and are introduced here through the chemical potentials. The total number of intensive variables and the number of independent intensive variables for a given system can be determined from the appropriate set of Gibbs–Duhem equations, consistent with any restrictions that may or may not be placed upon the system. The experimental and thermodynamic study of such a system then requires that a sufficient number of limitations be placed on the system so that it is univariant. Thus, we determine the dependence of one dependent variable on one independent variable, all other independent variables being held constant.