The P-v-TSurface
The state of a simple compressible substance is fixed by any two indepen-
dent, intensive properties. Once the two appropriate properties are fixed, all
the other properties become dependent properties. Remembering that any
equation with two independent variables in the form zz(x,y) represents a
surface in space, we can represent the P-v-Tbehavior of a substance as a
surface in space, as shown in Figs. 3–26 and 3–27. Here Tand vmay be
Chapter 3 | 125Critical
pointSolid Solid–LiquidSolid- Vapor
TemperatureVolumeTriple lineLiquid–
VaporVaporGasLiquidPressureFIGURE 3–26
P-v-Tsurface of a substance that
contractson freezing.Critical
pointSolidSolid–VaporTemperatureVolumeTriple lineLiquid–
VaporVaporGasLiquidPressureFIGURE 3–27
P-v-Tsurface of a substance that
expandson freezing (like water).