From the viewpoint of thermodynamics, when a body of fluid in the purely liquid phase is heated at a constant pressure, its temperature and volume will both increase, as reflected in the line from A to B in Figure 5.1. Point B represents the maximum temperature and volume of saturated liquid at the given pressure, and between points B and C, any additional heat added to the system results in a change from the liquid phase to the vapor phase. In this region, liquid and vapor phases coexist in equilibrium. The amount of heat required to convert a unit of mass from liquid to vapor is called the latent heat of vaporization h_fg (= h_g − h_f), where h_g and h_f are the enthalpy of saturated vapor and liquid, respectively. In this region, the addition of heat increases the vapor fraction and the volume of the mixture; however, the temperature of the two-phase mixture still remains at the saturation temperature. At point C, all the liquid has vaporized, and the vapor is now referred to as saturated vapor. Further heating will result in a temperature rise of the vapor (superheated vapor) along the constant-pressure line.