Since the volume parameterizes the points on the phase transition line between liquid and vapor (see Figure 12.7

one could use the density instead of the quality as the thermodynamic variable to characterize the mixed phase. Why do you suppose this is not done?

40 700 K 650 K 600 K 10 550 K Y 4 500 K 0.001 0.005 0.01 0.05 0.1 0.5 (a) specific volume [m/kg] critical 40 point 20 ideal gas isotherm saturation dome 10 pv = constant mixed phase saturated vapor saturated liquid 0.001 0.005 0.01 0.05 0.1 0.5 (b) specific volume [m/kg] 20 6 2. 64 2. pressure [MPa] pressure [MPa]