Question

If the boiling point difference between the two liquids is not much, then ________ method is used to separate them.

• A. simple distillation
• B. distillation under reduced pressure
• C. steam distillation
• D. fractional distillation
• E. differential extraction

Hint:

• Large boiling point difference → Simple distillation
• Small boiling point difference → Fractional distillation
• Heat-sensitive substances → Vacuum distillation
• Immiscible liquids → Steam distillation

Answer 1

The Correct Option is D

Concept:
Separation of liquid mixtures depends primarily on the difference in their boiling points. When two liquids have a large difference in boiling points (generally greater than 25–30$^\circ$C), simple distillation is sufficient because one component vaporizes much earlier than the other. However, when the boiling point difference is small, both liquids tend to vaporize simultaneously, making separation difficult. In such cases, fractional distillation is used. This method employs a fractionating column which allows repeated cycles of vaporization and condensation, effectively separating components based on slight differences in volatility.

Step 1: Understanding the limitation of simple distillation
In simple distillation, vapors rise directly from the liquid mixture to the condenser. If boiling points are close, both components evaporate together, resulting in poor separation and a mixed distillate.

Step 2: Role of fractional distillation
Fractional distillation introduces a fractionating column packed with materials (glass beads, plates, etc.) that increase surface area. This causes:
• Partial condensation of vapors
• Re-vaporization of more volatile component
• Gradual enrichment of vapor in the more volatile liquid Thus, by the time vapors reach the top, they are nearly pure in the component with lower boiling point.

Step 3: Working mechanism (Detailed)

• Mixture is heated → both components start vaporizing.
• Vapors enter fractionating column.
• Higher boiling component condenses earlier and flows back.
• Lower boiling component continues upward.
• Multiple equilibrium stages occur inside column. This repeated process leads to effective separation.

Step 4: Analysis of options
Option (A): Simple distillation
Used only when boiling point difference is large → Incorrect Option (B): Distillation under reduced pressure
Used for high boiling or heat-sensitive substances → Incorrect Option (C): Steam distillation
Used for immiscible liquids (like essential oils) → Incorrect Option (D): Fractional distillation
Best method when boiling points are close → Correct Option (E): Differential extraction
Used for separating based on solubility differences → Incorrect Final Conclusion:
For liquids having small difference in boiling points, the most efficient and appropriate method of separation is: \[ \boxed{\text{Fractional distillation}} \]