Consider a binary mixture of ethanol and water at low to moderate pressures. The vapor-liquid equilibrium (VLE) behavior is described by a modified form of Raoult's law with activity coefficients predicted by the Van Laar model .
This mixture presents a pressure-sensitive azeotrope, whose compositions and boiling points at are equal to mole % ethanol and . The azeotrope composition changes (i.e. its ethanol mole fraction increases) as pressure is decresased. The azeotrope will eventually disappear for very low pressures.
This Demonstration displays the contour plot of the relative volatility, , of this binary nonideal mixture. The dashed black curve gives the locus of the azeotope at various pressures (expressed in mmHg). As you drag the locator, the composition of the mixture, its pressure in mmHg, its boiling temperature in °C as well as the calculated relative volatility are displayed. The pressure-sensititve azeotrope is identified by the contour corresponding to (see the last snapshot for an example).