**Mass Balances for Binary Vapor-Liquid-Equilibrium**

This Demonstration illustrates what happens to a binary mixture initially in vapor-liquid equilibrium when a pure component is added at constant temperature and pressure. If both phases remain after the addition, it illustrates how the phase mole fractions (x_{A} is the mole fraction of A in the liquid, y_{A} is the mole fraction of A in the vapor) remain constant. If enough of either pure component is added, the mixture is transformed to a single phase. The initial vapor-liquid mixture contains 4 mol of A and 6 mol of B at a pressure of 130 kPa. Select "add A" or "add B" and use the slider to select the amount added. Adding a pure component changes the overall mole fraction of A in the mixture, as represented by the black point in the P-x-y diagram. If the overall mole fraction of A stays within the phase envelope, then x_{A} and y_{A} do not change. Selecting the "mole balance" button shows how this is possible. The size of a rectangle is proportional to the amount of that phase (blue is liquid, green is vapor). For example, adding A increases the vapor-to-liquid ratio, so some of the initial liquid vaporizes, and this amount is determined by a mass balance. For ease of visualization, A is shown being added as a vapor and B as a liquid, but the final equilibrium is the same if A is added as a liquid and B as a vapor because the system is isothermal. For example, if pure A were added as a liquid, it would vaporize to satisfy the mass balances.
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