Vaporus curve

Note that at the maximum or minimum point the liquidus and vaporus curves are coincident and thus the composition of the vapor is the same as that of the liquid. In other words, evaporation of those liquid solutions denoted by a and b in the above diagrams occurs without change of composition. This type of mixture is said to form a azeotrope. 

Toluene and benzene form liquid mixtures that are practically ideal and closely obey Raoult s law for partial pressure. For the binary system of these components, we can use the vapor pressures of the pure liquids to generate the liquidus and vaporus curves of the pressure-composition and temperature-composition phase diagram. The results are shown... 

In Fig. 13.6(a), the liquidus curve shows the relation between p and Xa for equilibrated liquid and gas phases at constant T, and the vaporus curve shows the relation between p and La under these conditions. We see that is a linear function of xa but not of ja-... 

A liquidus curve is also called a bubble-point curve or a boiling-point curve. Other names for a vaporus curve are dew-point curve and condensation curve. These curves are actually cross-sections of liquidus and vaporus surfaces in a three-dimensional T-p-ZA phase diagram, as shown in Fig. 13.7 on the next page. In this figure, the liquidus surface is in view at the front and the vaporus surface is hidden behind it. 

In practice, the data needed to generate the liquidus and vaporus curves of a nonideal binary system are usually obtained by allowing liquid mixtures of various compositions to boil in an equilibrium still at a fixed temperature or pressure. When the hquid and gas phases have become equihbrated, samples of each are withdrawn for analysis. The partial pressures shown in Fig. 13.8(a) were calculated from the experimental gas-phase compositions with the relations pa = y p and Pb = P — Pa-... 

If the system point is in a two-phase area of the phase diagram, we draw a horizontal tie line of constant temperature (on a temperature-composition phase diagram) or constant pressure (on a pressure-composition phase diagram). The lever rule applies. The position of the point at each end of the tie line, at the boundary of the two-phase area, gives the value of the composition variable of one of the phases and also the physical state of this phase either the state of an adjacent one-phase area, or the state of a phase of fixed composition when the boundary is a vertical line. Thus, a boundary that separates a two-phase area for phases a and P from a one-phase area for phase a is a curve that describes the composition of phase a as a funetion of T or p when it is in equilibrium with phase p. The curve is called a solidus, liquidus, or vaporus depending on whether phase a is a solid, liquid, or gas. 

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