# What other thermodynamic properties besides temperature and pressure may be graphed in phase diagrams?

##### 1 Answer

**TYPICAL PHASE DIAGRAMS**

Consider water's **phase diagram** as an unusual example of the ** typical kind** of phase diagram:

Here we have:

**Equilibrium**(coexistence) curves, where two phases coexist.- A
**triple point**, where three phases coexist. - A
**critical point**, where the liquid and vapor phase become a single superfluid phase.

**Phase transitions** would occur simply by crossing a curve, and while it is going on, you are still on the curve itself.

**PV PHASE DIAGRAMS**

A common alternative is a **phase diagram**, generally used to express the variation of molar volume of liquids compared to gases.

[Here, temperature IS a variable, but it extends out of the page, like a

Unlike in the usual

- There are equilibrium
**regions**(instead of equilibrium curves). - There is a triple
**line**(instead of a triple point). - The critical point lies on a
**double**-valued curve (instead of at the end of a single-valued curve).

A **phase transition** from liquid to vapor starts from the righthand intersection above the triple line and ends on the descending curve to the right.

*This clearly shows the increase in molar volume at constant pressure (and temperature) while vaporizing, as it should.*

- The
**starting**point of the transition marks a solution to the appropriate equation of state for the molar volume of the.*liquid* - The
**ending**point of the transition marks a solution to the appropriate equation of state for the molar volume of the.*gas*

This kind of graph finds more use by graphing the liquid-vapor curve *at various temperatures* to find the **critical natural variables**:

*The solid horizontal line is the aforementioned vaporization phase transition.*

When the cubic curve in the middle *coalesces* into a flat central slope, we have the **critical** temperature

This derivation of the constants

A

As you may see, in the second