Question

Which method is used to purify liquids having very high boiling points and liquids which decompose at or below their boiling point?

• A. Distillation
• B. Fractional distillation
• C. Distillation under reduced pressure
• D. Steam distillation

Hint:

- Simple Distillation: Large difference in B.P. (\(> 25-30^\circ\text{C}\)); separating liquid from non-volatile solids. - Fractional Distillation: Small difference in B.P.; uses a fractionating column. - Distillation under Reduced Pressure (Vacuum Distillation): For liquids with very high B.P. or those that decompose at/below their normal B.P. Lowering pressure lowers B.P. - Steam Distillation: For substances that are steam volatile and immiscible with water; allows distillation at temperatures below their B.P. Often used for extracting essential oils.

Answer 1

The Correct Option is C

- Distillation (Simple Distillation): Used to separate liquids with significantly different boiling points, or to separate a liquid from a non-volatile solid.
Not suitable for liquids with very high boiling points (requires very high temperatures) or those that decompose upon heating.
- Fractional Distillation: Used to separate a mixture of liquids whose boiling points are close to each other.
It involves a fractionating column.
Still not ideal for heat-sensitive compounds or very high boiling points.
- Distillation under Reduced Pressure (Vacuum Distillation): This method involves lowering the pressure above the liquid to be distilled.
Lowering the pressure reduces the boiling point of the liquid.
This allows liquids with very high boiling points to be distilled at lower, more manageable temperatures, and it prevents liquids that are thermally unstable from decomposing at their normal boiling points.
This is the suitable method.
- Steam Distillation: Used to purify liquids which are immiscible with water, volatile in steam, and have a high boiling point.
The mixture of water and the organic compound boils when the sum of their vapour pressures equals the atmospheric pressure.
This allows the organic compound to distill at a temperature lower than its normal boiling point.
It's particularly useful for temperature-sensitive organic compounds, but they must be steam volatile and water-insoluble.
For liquids with very high boiling points AND those that decompose at or below their normal boiling point, distillation under reduced pressure is the most appropriate method because it lowers the boiling temperature.
This matches option (3).