Q.Assertion [A] : The ortho and para isomers of nitrophenol can be separated by steam distillation.
Reason [R] : o-nitrophenol is steam volatile due to intra-molecular hydrogen bonding while p-nitrophenol is less volatile due to inter-molecular hydrogen bonding which causes the association of molecules.

Steam Distillation Volatility – From Intuition to Precision

Imagine you have a pot of water boiling on the stove. Now imagine you drop a few drops of a fragrant oil — say, clove oil — into the water. The oil doesn't dissolve; it floats as a separate layer. Yet, as the water boils, you smell the clove oil strongly in the steam. How did that oil, which boils at a much higher temperature than water, get carried into the vapour?

That is the core puzzle that steam distillation volatility explains.

The Intuition: Two Liquids That Don't Mix

When two immiscible liquids (like water and oil) are heated together, they do not behave like a single liquid. Each liquid exerts its own vapour pressure independently, as if the other weren't there. The total vapour pressure above the mixture is simply the sum of the two individual vapour pressures.

Now, boiling occurs when the total vapour pressure equals the surrounding atmospheric pressure. Because the two vapour pressures add up, the mixture reaches atmospheric pressure at a temperature lower than the boiling point of either pure liquid.

That is the key: the mixture boils at a temperature below 100°C (if water is one component) — often well below the boiling point of the organic compound. The organic compound, which would normally require a much higher temperature to boil, now gets carried over in the steam at this lower temperature.

The Precise Statement

When $P_{\text{total}}$ equals atmospheric pressure, the mixture boils. The temperature at which this happens is always less than the boiling point of pure water (100°C at 1 atm) and far less than the boiling point of the pure organic compound.

The vapour that distills over contains both water and the organic compound. The ratio of the masses of the two components in the distillate is given by:

where $P$ is the vapour pressure of each component at the distillation temperature, and $M$ is the molar mass.

Why This Matters for Exams

• Steam distillation volatility is not a property of the compound alone — it is a property of the mixture with water. A compound is "steam volatile" if it is immiscible with water and has a measurable vapour pressure at 100°C (or below).
• The compound does not need to have a low boiling point. Many high-boiling natural oils (like eugenol from clove, boiling point ~254°C) are steam volatile because they have enough vapour pressure at ~99°C to be carried over.
• The key condition: the compound must be immiscible with water. If it dissolves even slightly, the simple additive vapour pressure model breaks down. …