Q.By mistake, an alcohol (boiling point 97°C) was mixed with a hydrocarbon (boiling point 68°C). Suggest a suitable method to separate the two compounds. Explain the reason for your choice.

Concept: Separation by difference in boiling points (distillation)

When two miscible liquids have sufficiently different boiling points—typically a gap of at least 25°C—simple distillation cleanly separates them.

Here the boiling points are 68°C (hydrocarbon) and 97°C (alcohol), a difference of 29°C. On heating the mixture, the hydrocarbon vaporises first at ~68°C while the alcohol remains largely in the liquid phase. The hydrocarbon vapour passes into the condenser, cools, and collects as a separate distillate. Once the temperature rises toward 97°C, the alcohol distills over. …

When two miscible liquids have significantly different boiling points, we can exploit the fact that the more volatile component (lower boiling point) will vaporise preferentially when the mixture is heated. Simple distillation takes advantage of this property: as you heat the mixture, the component with the lower boiling point reaches its boiling point first, vaporises, and can be condensed and collected in a separate container, leaving the higher-boiling component behind.

The key question is always: how different must the boiling points be? For simple distillation to work cleanly, we need a separation of at least 25–30°C between the boiling points. Below that threshold, the vapors of both components mix too much, and you need the more sophisticated technique of fractional distillation (which uses a fractionating column to provide multiple vaporisation–condensation cycles).

Step-by-step separation

1. Identify the boiling points and their difference

   The hydrocarbon boils at 68°C and the alcohol at 97°C. The difference is $97 - 68 = 29\,°\text{C}$, which sits right at the boundary but is sufficient for simple distillation.

2. Set up a simple distillation apparatus

   Place the mixture in a round-bottom flask fitted with a thermometer (bulb at the side-arm level), a condenser, and a receiving flask. Heat the mixture gently.

3. Heat and collect the first fraction

   As the temperature approaches 68°C, the hydrocarbon begins to boil vigorously. Its vapor travels up, passes through the condenser (where it cools back to liquid), and drips into the receiving flask. The thermometer reading stays near 68°C during this phase.

4. Monitor the temperature

   Once most of the hydrocarbon has distilled over, the temperature will rise. When it approaches 97°C, the alcohol starts to boil. At this point, switch to a fresh receiving flask to collect the alcohol separately.

5. Result

   You now have the hydrocarbon (bp 68°C) in the first flask and the alcohol (bp 97°C) in the second, both in reasonably pure form. …