Which of the following can be obtained by fractional distillation?

Analysis

The question asks us to identify which of the provided processes utilizes fractional distillation as the primary method of separation. Fractional distillation is a technique used to separate a mixture of liquids that have different boiling points.

Concept Explanation

• Fractional Distillation: This process involves heating a mixture and collecting the components (fractions) as they vaporize at different temperatures. It is most commonly used for separating miscible liquids, such as the components of crude oil or the gases in liquid air.
• Liquid Air: Air is a mixture of gases (mainly nitrogen, oxygen, and argon). When air is liquefied under high pressure and low temperature, it becomes a mixture of liquids. Because these gases have different boiling points, they can be separated by fractional distillation.

Step-by-Step Solution

1. Evaluate Option A (Nitrogen from liquid air): Liquid air is a mixture of gases with different boiling points. Nitrogen has a boiling point of approximately -196°C, while oxygen boils at -183°C. Because their boiling points are distinct, fractional distillation is the standard industrial method used to separate them. This fits the definition perfectly.
2. Evaluate Option B (Sodium Chloride from sea water): This is typically achieved through evaporation or crystallization, not fractional distillation.
3. Evaluate Option C (Iodine from solution in carbon tetrachloride): This is a solid-liquid mixture. The separation is best achieved through solvent extraction or evaporation.
4. Evaluate Option D (Sulphur from solution in carbon disulphide): Similar to option C, this is a solid-liquid mixture. The sulphur is recovered by evaporation of the solvent (carbon disulphide).

Conclusion

Based on the principles of separation techniques, fractional distillation is specifically required for separating components of a liquid mixture with different boiling points, which is exactly how nitrogen is obtained from liquid air.

Exam Tip: Always remember the "Boiling Point Rule": If you see a question about separating liquids from a mixture (like crude oil or liquid air), the answer is almost always fractional distillation. If you are separating a solid dissolved in a liquid, look for evaporation or crystallization.