The variations of properties during phase-change processes are best studied and understood with the help of property diagrams such as the T-v, P-v, and P-T diagrams for pure substances.

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saturated liquid line
saturated vapor line
compressed liquid region
superheated vapor region
saturated liquid–vapor mixture region (wet region)

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Figure 3-17 Property diagrams of a pure substance.

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Figure 3-18 The pressure in a piston-cylinder device can be reduced by reducing the weight of the piston.

Extending the Diagrams to Include
the Solid Phase

Sublimation: Passing from the solid phase directly into the vapor phase

Phase Diagram

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The P-v-T surfaces present a great deal of information at once, but in a thermodynamic analysis it is more convenient to work with two-dimensional diagrams, such as the P-v and T-v diagrams.

Figure 3-23 P-v-T surface of a substance that contracts on freezing.

Figure 3-24 P-v-T surface of a substance that expands on freezing (like water).

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