Energy-volume-entropy surface (UVS) with Gibbs’ rolling plane (transparent) to mark the blue-yellow boundary of saturated vapor (green cross)-saturated liquid (red cross) states. The unstable region (usually colored red) is not differentiated in this drawing. The variables plotted have been nondimensionalized. The white cross marks the critical point. White curves denote constant molar entropy.

subscript N = molar value

underbars in these descriptions indicate extensive properties
subscript V = volume-scaled value

subscript C = critical-point value

Iowa State University