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given temperature. Thus a liquid with the composition xi will have a vapor pressure of 760 mm. at the temperature t2, and the vapor in equilibrium with it will have the composition yx = x2.

Starting with a mixture of the composition xh at a constant total pressure equal to 760 mm., and at a temperature below t2y there will be but one phase present, the liquid mixture of CC14 and CS2. As the temperature is raised, only a liquid phase will be present until the vertical line at Xi intersects the curve ABCy when a vapor phase of

0 Of 02 03 04 05 06 07 08 09 10 CCl4 Mol fraction C$2 C$2

Fig. 2-1. Boiling-point curve for CCI4-CS2 mixtures.

0 Of 02 03 04 05 06 07 08 09 10 CCl4 Mol fraction C$2 C$2

Fig. 2-1. Boiling-point curve for CCI4-CS2 mixtures.

the composition x2 will appear. Since there are now two phases and the pressure is fixed, there can be but one variable, temperature, and the composition of the phases will depend upon it. Let the temperature then be raised to some point ¿3 and the liquid and vapor compositions, being no longer independent variables, must change accordingly, which they do along the curves ABC and ADC, respectively, the liquid now having a composition x3, and the vapor in equilibrium with it a composition yz = rc4. It should be remembered that the quantity of CCI4 and CS2 in the system has not changed during this process; therefore, the change in the compositions of the liquid and the vapor includes such a corresponding change in the relative proportions of each phase that the total composition of the system remains the same, x%. Furthermore, the relative proportions of the liquid phase and the vapor phase, at the temperature tz, are as the distances FG and EF. It will be seen that, as the temperature is raised farther, the proportion of liquid phase decreases until, when the temperature reaches a point corresponding to the intersection of the vertical line xi and the curve ADC, which occurs at a temperature U, the vapor has the same composition as the original liquid, and the liquid phase disappears. At higher temperatures, there is but one phase, and the system again becomes trivariant, so that at constant pressure it is possible to vary

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