Experiments show that if the volume of a vessel containing a fixed amount of liquid is heated and expands at constant temperature, at a certain pressure, p s ( T ) , vapor, (denoted by dots at points f and g in Fig. 1) bubbles nucleate so the fluid is no longer homogeneous, but rather it has become a heterogeneous mixture of boiling liquid and condensing vapor. Gravity separates the boiling (saturated) liquid, v f = V f / N f , from the less dense condensing (saturated) vapor, v g = V g / N g > v f , that coexist at the same saturation temperature and pressure. As the heating continues the amount of vapor, N g increases and that of the liquid, N f = N − − N g , decreases. All the while the pressure, p s and temperature, T , remain constant and the volume V = V f + V g increases. In this situation the molar volume of the mixture is a weighted average of its components v = V / N = ( V f / N f ) ( N − − N g ) / N + ( V g / N g ) ( N g / N ) = v f ( 1 − − x ) + v g x where x = N g / N , the mole fraction of the vapor, 0 ≤ ≤ x ≤ ≤ 1 , increases continuously; however, the molar volume of the substance itself has only the largest possible stable value for its liquid state, and smallest possible stable value for its vapor state at the given p ( T ) . To repeat, although the mixture molar volume passes continuously from v f to v g (denoted by the dashed line in Fig. 1), the underlying fluid has a discontinuity in this property, and others as well. This equation of state of the mixture is called the lever rule.