The Phase Rule

The phase rule is the relationship between the variance, the number of components, and the number of phases at equilibrium for a system at any composition.

Here, the variance or number of degrees of freedom is the number of intensive variables that can be changed independently without altering the number of phases in equilibrium.

For example, in a single-component system, if only one phase is present, there are two degrees of freedom. This means that both pressure and temperature can be varied independently.

Suppose both solid and liquid phases are present at equilibrium. In that case, the degree of freedom equals one, suggesting that either pressure or temperature can be varied independently, allowing the other variable to adjust to maintain equilibrium. However, changing both pressure and temperature simultaneously would disturb the phase equilibrium.

When a system experiences equilibrium across its three phases, it has a variance value of zero, which implies that the system is invariant.

This exclusive state can only be achieved at a precise temperature and pressure distinct to the system, represented by the triple point.