Q.Explain why an organic liquid vaporises at a temperature below its boiling point in its steam distillation.

Concept: Steam Distillation Principle – immiscible liquids exert independent vapor pressures.

When two immiscible liquids (water and an organic compound) are heated together, each exerts its own vapor pressure independently of the other. The total vapor pressure above the mixture is simply the sum:

Boiling occurs when $P_{\text{total}}$ equals atmospheric pressure. Since both components contribute to the total pressure, this threshold is reached at a temperature lower than the boiling point of either pure liquid alone. For instance, if water contributes 600 mmHg and the organic liquid 160 mmHg at 95 °C, the mixture boils at 95 °C even though the organic compound alone might boil at 200 ° …

Why Steam Distillation Works: The Principle of Partial Pressures

When you heat a pure liquid, it boils when its vapour pressure equals atmospheric pressure. For many organic compounds—especially those that are heat-sensitive or have high boiling points—reaching that temperature would cause decomposition.

Steam distillation exploits a beautiful property of immiscible liquids: they don't "know" about each other in the liquid phase, so each exerts its own vapour pressure independently. The total pressure above the mixture is simply the sum of the individual vapour pressures.

This is the heart of the method. Because the two pressures add up, the mixture reaches atmospheric pressure (and therefore boils) at a temperature where neither component alone would boil.

Step-by-Step Reasoning

1. Immiscibility is key

   The organic liquid and water form two separate phases. Each liquid behaves as if it were alone in the container, developing its own vapour pressure according to its temperature. There's no molecular-level mixing to complicate things.

2. Dalton's Law applies

   The vapour phase above the mixture contains molecules of both substances. By Dalton's Law of Partial Pressures, the total pressure is the sum of what each liquid contributes independently:

where the superscript $\circ$ denotes the vapour pressure of the pure component at temperature $T$.

3. Boiling occurs when the sum reaches atmospheric pressure

   Normally, pure aniline (for example) boils at 184 °C because that's when $P_{\text{aniline}}^{\circ} = 1 \text{ atm}$. Pure water boils at 100 °C when $P_{\text{water}}^{\circ} = 1 \text{ atm}$.

   But in the mixture, suppose at 98 °C we have:

   • $P_{\text{aniline}}^{\circ}(98\,°\text{C}) = 0.04 \text{ atm}$
   • $P_{\text{water}}^{\circ}(98\,°\text{C}) \approx 0.96 \text{ atm}$

   Then $P_{\text{total}} = 1.00 \text{ atm}$, and the mixture boils at 98 °C—far below aniline's normal boiling point.

4. The organic liquid vaporises without reaching its boiling point

   Even though aniline's own vapour pressure is only 0.04 atm at 98 °C, it still evaporates into the vapour phase. The presence of water "helps" by contributing the remaining pressure needed to reach atmospheric pressure. The result: aniline distils over at a safe, low temperature. …