If a liquid compound decomposes at its boiling point, which method(s) can you choose for its purification. It

NCERT Exemplar · Q34

Q. If a liquid compound decomposes at its boiling point, which method(s) can you choose for its purification. It is known that the compound is stable at low pressure, steam volatile and insoluble in water.

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Concept understanding — Steam Distillation Principle

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

P_{\text{total}} = P_{\text{water}}^\circ + P_{\text{organic}}^\circ = P_{\text{atm}}

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.

Note

This is completely different from miscible liquids (like ethanol and water), where Raoult's law applies and the boiling point lies between the two pure boiling points. Immiscible liquids behave independently.

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:

\frac{m_{\text{organic}}}{m_{\text{water}}} = \frac{P_{\text{organic}}^\circ \times M_{\text{organic}}}{P_{\text{water}}^\circ \times M_{\text{water}}}

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. …

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A compound that decomposes at its normal boiling point but is stable at low pressure and steam-volatile can be purified by vacuum distillation or steam distillation, both of which lower the effective boiling point below the decomposition temperature.

When a liquid decomposes at its boiling point, conventional distillation at atmospheric pressure is impossible—you would destroy the compound before collecting it. The solution lies in understanding that boiling point is not a fixed property but depends on external pressure. By reducing the pressure or introducing steam, you can make the compound vaporise at a temperature well below its normal boiling point, safely beneath the decomposition threshold.

The compound's three properties—stability at low pressure, steam volatility, and water insolubility—point directly to two purification methods.

Why these methods work

The boiling point of any liquid is the temperature at which its vapor pressure equals the external pressure. At atmospheric pressure ( $760 \, \text{mm Hg}$ ), the compound reaches its vapor pressure of $760 \, \text{mm Hg}$ only at a temperature where it decomposes. But if we lower the external pressure or allow the compound to contribute only part of the total vapor pressure, it will boil at a much lower, safer temperature.

Method 1: Vacuum Distillation

Principle: Reduce the external pressure in the distillation apparatus using a vacuum pump. According to the Clausius–Clapeyron relation, vapor pressure increases exponentially with temperature. At reduced pressure—say $20$ – $50 \, \text{mm Hg}$ —the compound's vapor pressure reaches the new external pressure at a much lower temperature.
Application: The compound is heated under vacuum. Because it is stable at low pressure (given), it will not decompose at the reduced boiling point. The vapor is condensed and collected as pure liquid.
Advantage: Direct, efficient, and the compound remains dry. No water contamination.

Tip

Vacuum distillation typically lowers the boiling point by $100$ – $150 \, ^\circ\text{C}$ compared to atmospheric distillation, often enough to avoid decomposition.

Method 2: Steam Distillation

Principle: Pass steam through the liquid mixture. The total vapor pressure above the mixture is now the sum of the partial pressures of water and the organic compound:

P_{\text{total}} = P_{\text{water}} + P_{\text{compound}}

The mixture boils when $P_{\text{total}} = P_{\text{atmospheric}}$ , which happens at a temperature below the boiling point of either pure component. …

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Q.If a liquid compound decomposes at its boiling point, which method(s) can you choose for its purification. It is known that the compound is stable at low pressure, steam volatile and insoluble in water.

Why these methods work

Method 1: Vacuum Distillation

Method 2: Steam Distillation

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