Gibbs' Phase Rule
where P is the number of phases in equilibrium with each other and C is the number of components. Typical phases include various crystalline solids or compounds, liquids and gases. A system involving one pure chemical is an example of a one-component system. Two-component systems, such as mixtures of iron and carbon, have two chemically independent components. F is the number of degrees of freedom, which means the number of intensive properties such as temperature and pressure, which are independent of other intensive variables. In many condensed systems, such as those common in metallurgical and materials engineering, pressure is maintained at one atmosphere and the Gibbs Phase Rule reduces to F=C-P+1.
To learn more about the Gibbs' Phase Rule see MTGN381 Introduction to Phase Equilibria in Materials Systems taught by Prof. Speer.