A common graphic used to illustrate the relationship of substances in their phase states as a function of pressure and temperature is the P-T diagram. It is also referred to as a phase state diagram or a phase change diagram. The asterisk in the P-T diagram below identifies a pressure-temperature combination that is known as the triple point, which is where the substance can simultaneously exist as a solid, a liquid, or a vapor.
P-T Diagram for a Substance That Expands On Freezing
|Liquid||Solid||Fusion (aka Freezing)|
| Critical Point:||Pure vapor phase has same properties as pure liquid phase at same temperature and pressure. It is the highest temperature at which a gas can be liquefied, regardless of pressure.|
| Triple Point:||Possible to maintain equilibrium mixture of all three phases.|
Posted March 13, 2015