This Demonstration shows the phase equilibrium of a binary system of two liquids, A and B, that are only partially miscible. Vapor-liquid equilibrium, liquid-liquid equilibrium, and vapor-liquid-liquid equilibrium regions are present on the phase diagram because of the partial miscibility. The amounts of each of the three phases present for the temperature and mole fraction of component B represented by the black point (in the
diagram) are shown on the right in the bar graph. You can vary the location of the black point by changing the mole fraction of component B and the heat added. The heat added changes the temperature, except when on the horizontal line at about 77
C, when three phases can exist in equilibrium: an
), and a vapor (
). The relative amounts of each phase are determined from a mole balance. One phase must evaporate (or condense if removing heat) before the temperature increases (or decreases). The amount of heat added is used to illustrate the system behavior and to show how one point on the diagram at about 77
C can represent multiple phase conditions. The amount of heat added is not meant to be an accurate value for a real system, but is used to illustrate what phases can be present. The bar graph also shows the mole fraction of component B in each phase, and the mole fraction is set to zero when that phase is not present. In two phase regions, the lever rule is used to determine the amounts of each phase. The
diagram also locates the mole fractions in equilibrium in the two phase regions using vertical dashed lines.