Q.What do you know about Steam Distillation?

Steam Distillation: The Intuition First

Imagine you have a pot of water with a layer of oil floating on top. You want to separate that oil, but you can't just boil it — many oils decompose or catch fire long before they reach their own boiling point. Yet, if you heat the water underneath, something interesting happens: the oil starts to vaporise along with the steam, at a temperature far below its normal boiling point. That is steam distillation in action.

The core trick is this: two immiscible liquids (like oil and water) boil together at a temperature lower than either one's individual boiling point. The mixture boils when the sum of their vapour pressures equals the atmospheric pressure — not when either one alone does.

The Precise Principle

For two immiscible liquids, the total vapour pressure above the mixture is simply the sum of the vapour pressures of the pure components (each exerts its own vapour pressure independently, as if the other weren't there). Boiling occurs when this sum equals the external pressure.

Since each vapour pressure is always less than $P_{\text{atm}}$ at any temperature below its own boiling point, the mixture boils at a temperature lower than the boiling point of either pure liquid.

Why This Matters for Exam Context

The key consequence: you can distill a high-boiling, heat-sensitive organic compound at around 100 °C (or lower) by simply passing steam through it. The organic compound never gets hot enough to decompose.

The composition of the distillate (the ratio of water to organic liquid collected) is fixed by their vapour pressures and molecular weights:

This is derived from the ideal gas law — the number of moles of each in the vapour is proportional to its partial pressure, and mass = moles × molar mass.

A Concrete Example

Take aniline (boiling point 184 °C) and water. At 98.4 °C:

• Vapour pressure of water: 717 mm Hg
• Vapour pressure of aniline: 43 mm Hg
• Total: 760 mm Hg → mixture boils at 98.4 °C

The distillate contains about 1 g of aniline for every 5 g of water — enough to collect the compound without ever heating it above 100 °C. …