Problem 37 - A Complete Resource Book in Chemistry for JEE Main

Question

Absolute alcohol cannot be obtained by simple fractionation of a solution of water and alcohol because (a) ethanol molecules are solvated (b) boiling points are very near (c) ethanol remains dissolved in water (d) they form a constant boiling mixture

Step-by-Step Solution

Answer

Due to azeotrope formation, ethanol and water cannot be separated by simple fractionation.

Step 1: Understanding the Question

The question is asking why absolute alcohol cannot be obtained through simple fractionation. Fractionation is a process of separating a mixture into its components by using differences in boiling points.

Step 2: Analyzing Fractional Distillation

In fractional distillation, components are separated based on boiling point differences. Ethanol and water both have distinct boiling points: 78.37°C for ethanol and 100°C for water.

Step 3: Identify Characteristics of a Constant Boiling Mixture

A constant boiling mixture, or azeotrope, behaves as a single substance when boiled. Ethanol and water form an azeotrope at about 95.6% ethanol and 4.4% water, boiling at a lower temperature than either pure substance alone, making further separation by distillation impossible.

Step 4: Considering Other Options

Evaluate other options like solvation of ethanol or ethanol being dissolved in water. While ethanol is indeed solvated in water, and ethanol remains in solution due to its solubility, these factors do not address the distillation process directly.

Step 5: Drawing Conclusion

Due to the formation of an azeotrope, ethanol and water cannot be completely separated by simple fractionation. This azeotropic behavior results in ethanol and water distilling together at a constant ratio and boiling point.

Key Concepts

Fractional Distillation

Fractional distillation is a technique used to separate a mixture into its individual components based on differences in their boiling points. When a liquid mixture is heated, the component with the lower boiling point will vaporize first. This vapor is then condensed back into its liquid form and collected. The process is repeated across different temperature levels using a column known as a fractionating column. However, when it comes to certain mixtures like ethanol and water, fractional distillation has its limitations.

Ethanol and water, when mixed, form a special type of mixture known as an azeotrope. This means that they boil together as a single compound rather than separately, making complete separation by distillation challenging. As a result, despite ethanol having a lower boiling point than water, the two liquids evaporate and condense together at a certain ratio.

Boiling Points

Every liquid has a specific boiling point at which it changes from a liquid to a gaseous state. Ethanol and water each have their unique boiling points: 78.37°C for ethanol and 100°C for water. In a standard mixture of these substances, you might expect to separate them based on these differences. However, it is not as straightforward.
Ethanol and water together form an azeotropic mixture at a certain concentration, which introduces a complication. This mixture boils at approximately 78.1°C, a value lower than that of pure ethanol or water. Consequently, the typical boiling point separation method is foiled by the azeotropic characteristic, where the mixture boils at a different composite and integrates both substances.

Ethanol-Water Mixture

The ethanol-water mixture is a classic example of a binary azeotrope, which presents a unique challenge in separation processes. When you mix ethanol and water, they form an azeotrope at 95.6% ethanol and 4.4% water. This azeotropic mixture has its own properties and behaves differently than the individual components.

In industrial applications, separating these components requires more than just simple fractional distillation due to the azeotrope. Techniques such as pressure-swing distillation or the addition of another substance to break the azeotropic formation are often employed. Understanding how these mixtures interact at the molecular level is crucial for developing efficient separation techniques. This is why obtaining pure ethanol through simple means remains a significant challenge.