Which of the following is an example of minimum boiling azeotrope?

Ethanol and acetone

The correct answer is Option 4. Ethanol and acetone.

Let us dive deeper into why ethanol and acetone form a minimum boiling azeotrope and clarify the concept of azeotropes.

An azeotrope is a mixture of two or more liquids that has a constant boiling point and composition throughout the distillation process. This means that when you distill an azeotropic mixture, the vapor produced has the same composition as the liquid mixture, making it difficult to separate the components by simple distillation.

Types of Azeotropes

Minimum Boiling Azeotropes: These mixtures boil at a lower temperature than any of their individual components. They typically exhibit positive deviations from Raoult's Law. In these cases, the interactions between the different molecules in the mixture are such that they enhance the volatility of the mixture compared to the individual components.

Maximum Boiling Azeotropes:  These mixtures boil at a higher temperature than any of their individual components. They often show negative deviations from Raoult's Law, meaning the mixture is less volatile than the pure components.

Ethanol and Acetone

Interactions:  Ethanol and acetone can form hydrogen bonds between the hydroxyl group of ethanol and the carbonyl group of acetone. These interactions lead to a stronger attraction between the two molecules compared to the individual molecules of ethanol or acetone alone.

Boiling Point: When ethanol and acetone are mixed, the resulting mixture has a boiling point that is lower than either pure ethanol (approximately 78.37°C) or pure acetone (approximately 56.05°C). This boiling point is typically around 56.5°C for the azeotropic mixture, making it a minimum boiling azeotrope.

Composition: The azeotropic composition of ethanol and acetone is roughly 70% ethanol and 30% acetone by weight. This composition remains constant during distillation.

Comparison with Other Options

Phenol and Aniline: These do not form a minimum boiling azeotrope; instead, they exhibit strong intermolecular hydrogen bonding that leads to a higher boiling point.

Chloroform and Acetone: While this pair is often mentioned in discussions of azeotropes, they form a different type of mixture and are less commonly cited as the classic example compared to ethanol and acetone.

Nitric Acid and Water: This combination forms a maximum boiling azeotrope due to strong interactions and resistance to vaporization.

Conclusion

Ethanol and acetone are widely recognized for forming a minimum boiling azeotrope, where the boiling point of the mixture is lower than that of either pure component. This characteristic makes their separation through simple distillation challenging, and they exemplify the concept of positive deviations from Raoult's Law