A rule called the Gibbs phase rule tells us how many phases can be in coexistence for a given substance or mixture. The rule arises from the fact that, to be in coexistence, a constraint exists that the chemical potentials of each component in each phase must be equal. After some math, one can find a relationship between the number of components of a system, the number of free variables (such as temperature, pressure, or the fraction of a given component present in a mixture), and the number of phases that can be in coexistence. This relationship is:

where F is the number of degrees of freedom, C is the number of independent components, and P is the number of phases.