### The State Postulate and Gibbs Phase Rule

##### Definition:

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?

##### Definition:

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

##### Note:

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.

##### Definition:

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)?