5.6 Azeotropic Point

Figure 5.06 is a closeup view of the ethanol-rich portion of the water-ethanol diagram of Figure 5.02. The boling and condensation curves for water-ethanol mixtures
Water-Ethanol Phase Diagram

Figure 5.06. Azeotropic point on the water-ethanol phase diagram. A water-ethanol mixture that is 95.6 weight percent ethanol boils to a vapor of the same composition at 78.17°C. The boling curve and the condensation curve both meet at the azeotropic point. Click on the diagram to see a larger version with more information.

both have minima at the same temperature (78.17°C) and composition (95.6 weight percent ethanol). This means that a 95.6 wt.% ethanol mixture boils to a vapor of the same composition. This special solution is called an azeotrope, and the boiling point is an azeotropic point. Solutions with more ethanol than the azetrope produce vapor that has less ethanol than the liquid upon boiling. Solutions with less ethanol than the azetrope produce vapor that has more ethanol than the liquid upon boiling. The azeotropic point is a limit on the possible change of composition by distillation.

Some solid solution minerals have azeotropic points (minima) where their liquidus and solidus curves meet. Change of the compositions of these minerals through melting or crystallization is limited by the azeotrope composition. Alkali feldspar is a common mineral with a minimum (azeotropic point) on its liquidus at 1 bar pressure.