Two liquids (A) and (B) can be separated by the method of fractional distillation. The boiling point of liquid

NCERT Exemplar · Q63

Q.Two liquids (A) and (B) can be separated by the method of fractional distillation. The boiling point of liquid (A) is less than boiling point of liquid (B). Which of the liquids do you expect to come out first in the distillate? Explain.

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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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In fractional distillation, the component with the lower boiling point vaporises first and is collected earliest in the distillate. Liquid A will come out first because it has the lower boiling point.

Why Fractional Distillation Separates by Boiling Point

Fractional distillation exploits a fundamental property: different liquids vaporise at different temperatures. When you heat a mixture, the component with the lower boiling point reaches its vaporisation temperature first, converts to vapor preferentially, rises through the fractionating column, and condenses in the receiver before the higher-boiling component has a chance to vaporise significantly.

The fractionating column provides multiple vaporisation-condensation cycles. As vapor rises and encounters cooler surfaces, the higher-boiling component (which is less volatile) condenses and drips back down, while the lower-boiling component (more volatile) continues upward. This repeated separation sharpens the distinction, ensuring that what reaches the top and distills over is enriched in the more volatile substance.

Step-by-Step Reasoning

Initial heating of the mixture

When the mixture of A and B is heated, both liquids begin to gain thermal energy. However, liquid A requires less energy to overcome intermolecular forces because it has a lower boiling point.
Vaporisation of the lower-boiling component

As the temperature approaches the boiling point of A (let's call it $T_A$ ), liquid A starts vaporising rapidly. Since $T_A < T_B$ (where $T_B$ is the boiling point of B), liquid A converts to vapor while most of liquid B remains in the liquid phase.
Selective ascent through the fractionating column

The vapor, now rich in component A, rises through the column. Each time it encounters a cooler region, a small amount condenses. Because B has a higher boiling point, any B vapor present condenses preferentially, dripping back into the flask. Component A vapor, being more volatile, re-vaporises and continues upward.
Collection in the distillate …

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Q.Two liquids (A) and (B) can be separated by the method of fractional distillation. The boiling point of liquid (A) is less than boiling point of liquid (B). Which of the liquids do you expect to come out first in the distillate? Explain.

Why Fractional Distillation Separates by Boiling Point

Step-by-Step Reasoning

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