The Major Features of a Phase Diagram

The triple point of a substance such as water can he reproduced anywhere to calibrate a thermometer or pressure gauge with a known temperature and pressure. 

Hie Triple Point The triple point in a phase diagram represents the unique set of conditions at which the three states are equally stable and in equilibrium. In the phase diagram for water, the triple point occurs at 0.0098 °C and 4.58 torr. Under these unique conditions (and only under these conditions), the solid, liquid, and gas states of water are equally stable and will coexist in equilibrium. 

The Critical Point As we discussed in Section 11.5, at the altical temperature and pressure, the liquid and gas states coalesce into a supercritical fluid. The critical point in a phase diagram represents the temperature and pressure above which a supercritical fluid exists. 

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