Even though the phase change produces a mathematical discontinuity in the homogeneous fluid properties, for example v , there is no physical discontinuity. As the liquid begins to vaporize the fluid becomes a heterogeneous mixture of liquid and vapor whose molar volume varies continuously from v f to v g according to the equation of state v = v f + x ( v g − − v f ) x = N g / ( N f + N g ) where 0 ≤ ≤ x ≤ ≤ 1 is the mole fraction of the vapor. This equation is called the lever rule and applies to other properties as well. The states it represents form a horizontal line connecting the same colored dots on an isotherm, but not shown in Fig. 1 as noted already since it is a distinct equation of state for the heterogeneous combination of liquid and vapor components.