Critical Regions. At very high pressures special phenomena associated with the critical region are encountered in vapor-liquid equilibria. If the vapor pressure of a pure component is plotted vs. the temperature, a line concave upward is obtained. This line terminates at the critical point. Conditions below the line in region A, Fig. 4-1,
Fig. 4-1. Typical pressure-temperature curves.
correspond to all vapor and no liquid. The line represents conditions under which the vapor and liquid are in equilibrium. Conditions above the line in region B represent all liquid. In region C the state of the substance is in question since it is possible to obtain either vapor or liquid without a change in phase. If a given binary mixture is plotted in the same way, similar conditions are attained except that a loop region is obtained for a mixture of given composition instead of a single line, Fig. 4-1. The upper line of the loop represents the bubble-point curve, t.e., the condition under which the mixture first forms vapor. The lower side of the loop is the dew point curve, i.e., the condition under which the mixture begins to condense. In the case
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