Critical phenomena of mixtures

If we run the same experiment, going up in pressure, at Tc we find that at Pc the two phases become identical and the interface between them vanishes. This is the critical point of 

FIGURE 10.4 Typical experimental result for one specific mixture of two species, showing bubble-point and dew-point curves, critical point, cricondentherm and cricondenbar, and the paths for retrograde condensation or vaporization of the first and second kinds. 

FIGURE 10,5 Bubble- and dew-point curves for a variety of mixtures of ethane and n-heptane. See the text for a description of this figure. (Reprinted with permission from Kay, W. B. Liquid-vapor phase equilibrium relations in the ethane-n-heptane system, Ind Eng. Chem. 30 459-465. Copyright (1938) American Chemical Society.) 

The L-R rule makes little change for the ethane, which practically behaves as an ideal gas. But for -heptane the calculated value is low enough that it makes a 

Ethane is above its critical temperature it could not exist as a pure liquid at this temperature. The temperature is low enough and the pressure high enough that M-heptane could not exist as a vapor at this temperature and pressure. That combination of values makes this a severe test of Raoult s law and the L-R mle. The results shown here demonstrate fair, but not good, agreement between calculation and experiment. 

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Naturally, the critical phenomena of mixtures are more complex and varied than those of single pure species. Here we consider only the simplest (and most common) type. The experimental data are mostly obtained in devices like that shown in Figure 10.3. 

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