Thermodynamics

cen84959_ch03.qxd 4/1/05 12:31 PM Page 125PressurePressureSolid–LiquidLiquidGasGasThe P-v-T SurfaceThe state of a simple compressible substance is fixed by any two independent,intensive properties. Once the two appropriate properties are fixed, allthe other properties become dependent properties. Remembering that anyequation with two independent variables in the form z z(x, y) represents asurface in space, we can represent the P-v-T behavior of a substance as asurface in space, as shown in Figs. 3–26 and 3–27. Here T and v may beChapter 3 | 125SolidCriticalpointLiquid–Vapor VaporTriple lineVolumeSolid–VaporTemperatureFIGURE 3–26P-v-T surface of a substance thatcontracts on freezing.LiquidCriticalpointSolidLiquid–VaporTriple lineSolid–VaporVaporVolumeTemperatureFIGURE 3–27P-v-T surface of a substance thatexpands on freezing (like water).