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Unformatted text preview: he two phases. A wet T-shirt dries much quicker in dry air than it does in humid air. In fact, it does not dry at all if the relative humidity of the environment is 100 percent. In this case, there is no transformation from the liquid phase to the vapor phase, and the two phases are in phase equilibrium. The conditions of phase equilibrium change, however, if the temperature or the pressure is changed. Therefore, we examine phase equilibrium at a specified temperature and pressure. Phase Equilibrium for a Single-Component System
T, P VAPOR mg LIQUID mf The equilibrium criterion for two phases of a pure substance such as water is easily developed by considering a mixture of saturated liquid and saturated vapor in equilibrium at a specified temperature and pressure, such as that shown in Fig. 1618. The total Gibbs function of this mixture is
G mf gf mg gg FIGURE 1618 A liquidvapor mixture in equilibrium at a constant temperature and pressure. where gf and gg are the Gibbs functions of the liquid and vapor phases per unit mass, respectively. Now imagine a disturbance during which a differential amount of liquid dmf evaporates at constant temperature and pressure. The change in the total Gibbs function during this disturbance is
1dG 2 T,P...
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- Summer '08