trichloromethane/propanone and nitric acid/water mixtures. Phase diagrams showing a minimum (figure) indicate that the mixture is destabilized relative to the ideal solution, A-B interactions then being unfavourable. For such mixtures GE is positive (less favourable to mixing than ideal), and there may be contributions from both enthalpy and entropy effects. Examples include dioxane/water and ethanol/water mixtures. The system shown in the figure below is also azeotropic, but shows its azeotropy in a different way. Suppose we start with a mixture of composition a1, and follow the changes in the composition of the vapour that raises trough a fractionatimg column. The mixture boils at a2 to give a vapour of composition a'2. This vapour condenses in the column to a liquid of the same composition (a3). That liquid reaches equilibrium with its vapour at a3', which condenses higher up the tube to give a liquid of the same composition, which we now call a4. The fractionation therefore shifts the vapour towards the azeotropic composition at b, but not beyond, and the azeotropic vapour emerges from the top of the column. An example is ethanol/water, which boils
trichloromethane/propanone and nitric acid/water mixtures. Phase diagrams showing a minimum (figure) indicate that the mixture is destabilized relative to the ideal solution, A-B interactions then being unfavourable. For such mixtures GE is positive (less favourable to mixing than ideal), and there may be contributions from both enthalpy and entropy effects. Examples include dioxane/water and ethanol/water mixtures. The system shown in the figure below is also azeotropic, but shows its azeotropy in a different way. Suppose we start with a mixture of composition a1, and follow the changes in the composition of the vapour that raises trough a fractionatimg column. The mixture boils at a2 to give a vapour of composition a'2. This vapour condenses in the column to a liquid of the same composition (a3). That liquid reaches equilibrium with its vapour at a3', which condenses higher up the tube to give a liquid of the same composition, which we now call a4. The fractionation therefore shifts the vapour towards the azeotropic composition at b, but not beyond, and the azeotropic vapour emerges from the top of the column. An example is ethanol/water, which boils
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