Advanced Topic 305 of 1102 States of Matter & Solutions
Azeotropes
Mixtures that boil at constant composition and cannot be separated by simple distillation.
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In-depth explanation of Azeotropes
An azeotrope is a mixture that boils at a constant composition, where the vapour has the same composition as the liquid. Because the vapour and liquid share the same composition at this point, simple distillation cannot separate the components beyond the azeotropic composition. Azeotropes occur in systems with large deviations from Raoult's law. Systems with positive deviation give minimum-boiling azeotropes, while systems with negative deviation give maximum-boiling azeotropes. Special techniques such as pressure-swing distillation or azeotropic distillation with an added entrainer are used to break them.
Going deeper: Azeotropes
Key ideas: Types: minimum-boiling (positive deviation, like ethanol-water at 95.6 percent) and maximum-boiling (negative deviation, like HCl-water at about 20.2 percent HCl, boiling at 108.6 C). Azeotropes act like pure liquids at that pressure but changing pressure shifts composition. Absolute ethanol is made by adding an entrainer such as benzene or using molecular sieves.
Common mistakes and connections: A typical mistake is thinking azeotropes are compounds; the composition changes with pressure. Another is expecting fractional distillation to give 100 percent ethanol from a dilute solution. It only gets up to the azeotrope. Boiling point at the azeotrope is a minimum or maximum on the diagram. Links to Raoult's law deviations and distillation.
Visualization of Azeotropes
Azeotropes occur at extremes of the vapour pressure curve: 95.6% ethanol-water boils at constant composition, so distillation cannot separate it further. The interactive lesson shows this as a live animation or chart.
Worked example and use case
The ethanol-water azeotrope boils at about 78.2°C and contains 95.6 percent ethanol by mass, which is why pure ethanol cannot be obtained by ordinary distillation of wine.
Frequently asked questions about Azeotropes
What is Azeotropes?
Mixtures that boil at constant composition and cannot be separated by simple distillation.
Is Azeotropes a beginner, intermediate or advanced topic?
Azeotropes is a advanced topic in States of Matter & Solutions. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Azeotropes?
Study these earlier topics in States of Matter & Solutions first: Temperature and Solubility, Raoult's Law, Ideal and Non-ideal Solutions.
What comes after Azeotropes?
The next topics in States of Matter & Solutions are Colligative Properties and Boiling Point Elevation.
Can I practise Azeotropes with quizzes?
Yes. The interactive lesson includes a self-check question on Azeotropes, and the States of Matter & Solutions quiz draws on every topic in this area.
Where Azeotropes fits in States of Matter & Solutions
Azeotropes is topic 86 of 109 in States of Matter & Solutions.