Question

The technique is used to purify liquid having very high b.p and decompose at or below this boiling point is

Hint:

Match the technique to the problem:
Decomposes at boiling point $\rightarrow$ Vacuum distillation
Immiscible with water & steam volatile $\rightarrow$ Steam distillation
Small difference in boiling points $\rightarrow$ Fractional distillation

Answer 1

Step 1: Understanding the Concept:
Simple distillation relies on heating a liquid to its boiling point. A liquid boils when its vapor pressure equals the external atmospheric pressure. However, some liquids decompose at temperatures below their normal boiling point.

Step 2: Key Formula or Approach:
The approach is factual recall of separation techniques. To boil such liquids without decomposing them, we must lower their boiling point. This is achieved by lowering the external pressure acting on the liquid surface.

Step 3: Detailed Explanation:
The technique used is Distillation under reduced pressure, also known as vacuum distillation.
By using a vacuum pump to reduce the pressure inside the distillation apparatus, the vapor pressure of the liquid will equal the external pressure at a much lower temperature. Consequently, the liquid boils and distills at a temperature significantly lower than its normal boiling point, thus avoiding thermal decomposition.
A classic example is the purification of glycerol, which decomposes at its normal boiling point ($290^\circ\text{C}$) but can be safely distilled without decomposition at a lower temperature under reduced pressure.

Step 4: Final Answer:
The technique is distillation under reduced pressure (vacuum distillation).