Q.A liquid with high boiling point decomposes on simple distillation but it can be steam distilled for its purification. Explain how is it possible?

Concept: Steam distillation exploits the principle that immiscible liquids boil when the sum of their individual vapor pressures equals atmospheric pressure, allowing distillation below the boiling point of either pure component.

When two immiscible liquids are heated together, each exerts its own vapor pressure independently. The mixture boils when:

For a high-boiling organic liquid (say, boiling point 200 °C at 1 atm), simple distillation would require heating to 200 °C—often causing thermal decomposition. In steam distillation, water contributes significant vapor pressure even at lower temperatures. The mixture therefore boils well below 100 °C (typically 95–99 °C), because $P_{\text{liquid}}$ need only contribute the small remainder to reach $P_{\text{atm}}$ alongside $P_{\text{water}}$. …

Why Steam Distillation Succeeds Where Simple Distillation Fails

When a liquid has a very high boiling point, heating it to that temperature during simple distillation often causes thermal decomposition—the molecules break apart before they can vaporise intact. Steam distillation elegantly sidesteps this problem by exploiting a fundamental principle: immiscible liquids in contact exert their vapour pressures independently.

The key insight is that the mixture doesn't need to reach the boiling point of either pure component. Instead, it boils when the combined vapour pressure reaches atmospheric pressure, and this happens at a temperature lower than the boiling point of either liquid alone.

The Mechanism, Step by Step

1. Two immiscible liquids, two independent vapour pressures When water (steam) and the organic liquid are together but immiscible, each behaves as if the other weren't there. The water molecules exert their own vapour pressure $p_{\text{water}}$, and the organic molecules exert theirs, $p_{\text{organic}}$. The total pressure above the mixture is simply:

2. Boiling occurs when the sum reaches atmospheric pressure

   The mixture boils when $P_{\text{total}} = P_{\text{atm}}$ (typically 760 mm Hg). Crucially, this condition is met at a temperature below 100 °C—in fact, below the boiling point of water—because both liquids contribute to the total pressure. You don't need to heat the organic liquid anywhere near its own (dangerously high) boiling point.

3. The organic liquid vaporises at a safe temperature

   Even though the organic compound might normally boil at, say, 250 °C (where it decomposes), in the presence of steam it vaporises at perhaps 95–99 °C. At this lower temperature the molecules remain intact. The steam "carries" the organic vapour over into the condenser.

4. Separation after condensation

   The vapours condense together, but because the liquids are immiscible they separate into two layers in the receiving flask. You can then simply decant or use a separating funnel to isolate the purified organic liquid.