TP: The State postulate and the Gibbs phase rule

The State Postulate and Gibbs Phase Rule


The state postulate says that the thermodynamic state of a system containing a pure substance may be uniquely defined by knowing two (independent) values of its (the system's)  intensive variables.

Test Yourself:

If you knew the temperature and pressure of an ideal gas could you calculate the specific volume?


The Gibbs phase rule tells us that the intensive properties of a given phase may be uniquely identified by knowing $D$ (independent) intensive variables where D is given by

$D = c - \pi + 2$

c = # of components
$\pi$ = # of phases


The T and P in a multiphase mixture would count toward both phases!

However, in order to constrain the state of the system we need to know independent intensive variables, so we need to be careful with this information.


The quality of a vapor-liquid mixture is the mole fraction of the mixture that is vapor.

Test Yourself:

How many intensive variables do you need to know to determine the state of melting ice? How about a mixture of melting ice and some of the (already melted water)?