# Gibbs Phase Rule

Previously, we used "degrees of freedom" to denote the number of things we needed to know in order for our material balance problem to be completely specified.

The Gibbs phase rule tells you how many intensive variables must be known in order to completely specify a system! (It tells you the total number of degrees of freedom.) Recall the definition of an intensive variable:

##### DEFINITION:

An intensive variable is one which does not depend on system size (like temperature, pressure, or specific volume).

##### DEFINITION:

The Gibbs Phase Rule states that the number of degrees of freedom is 2 plus the number of components minus the number of phase, i.e., DF = 2 + c - $\pi$

Prove to yourself that the number of degrees of freedom of boiling water is 1.

##### Outcome:

Use Gibbs phase rule to determine how much information is necessary to specify the thermodynamic state of a system

##### Test Yourself

How many degrees of freedom are there at the triple point?!