(26-1) |

The Gibbs phase rule is a very useful equation because it put precise limits on the number of phases that can be simultaneously in equilibrium for a given number of components.

What does Equation 26-1 mean? Consider the following example of a single component (pure) phase diagram .

Consider a single-phase region:

This implies that two variables ( and ) can be changed independently (i.e., pick any and ) and a single phase remains in equilibrium.

Consider where two phases are in equilibrium:

,

There is only one degree of freedom-for the two
phases to remain in equilibrium, one
variable can be changed freely (for instance,
) but then
the change in the other variable (i.e.,
) must depend
on the change of the free variables:

Finally, consider where three phases are in
equilibrium then:

.

There can be no change any variable that maintains three phase equilibrium.