5.3 Water Ethanol Diagram

Modern experiments on boiling and condensation of water-ethanol mixtures are summarized as a phase diagram in Figure 5.02. The diagram is divided into three regions by two saturation curves. The lower (boiling) curve marks the temperatures of boiling (vapor separation) as a function of the alcohol content of the liquids.
Water-Ethanol Phase Diagram

Figure 5.02. Water-ethanol phase diagram. Phase relations for water-ethanol mixtures at various temperatures at 1 bar pressure. Click on the diagram for a larger, interactive version with more information.

The boiling curve shows that adding alcohol to water reduces the boiling temperatures. At equilibrium, liquids that plot along the boiling curve are saturated with vapor. The upper (condensation) curve marks the temperatures of condensation (liquid separation) as a function of the alcohol content of the vapor. At equilibrium, vapors that plot on the condensation curve are saturated with liquid. The region between the two curves indicates temperatures and compositions for which there are two phases in equilibrium, liquid and vapor. The proportions of the two phases can be determined with the lever rule. Click on the diagram to see a larger, interactive version.

You can see from the water-ethanol diagram that the vapor phase produced when a liquid boils is much richer in ethanol than the boiling liquid. On the larger version of Figure 5.02 follow the tie lines to see the vapor composition in equilibrium with each liquid composition. Click on the "Show Phase %" button and mouseover the diagram to see the lever rule applied to the two-phase mixtures. Because the vapor is more alcohol-rich than the boiling liquid, the liquid will become less rich in alcohol (more H2O rich) as the boiling proceeds. If the vapor is confined and all the liquid boiled to vapor, the vapor will have the same composition as the original liquid. In a still, the alcohol-richer vapor is removed as it is formed and condensed separately, so the liquid left behind is less alcohol-rich.

If a water-ethanol mixture that is 50 wt% ethanol is heated to 85°C, what will be the wt% ethanol in the vapor?
You may useFigure 5.02 and its "Coordinates" and "Show Phase %" buttons to help you find the answer this question. Press "Enter" after you type in the number.

 To get large volumes of alcohol-rich liquid, commercial stills have an iterative boiling-condensation process. The minimum on the boiling curve is an azeotropic point, which limits the weight percent ethanol that can be produced. We will return to azeotropic points later, but let's move on to melting minerals with solid solution.