Q.Components of a binary mixture of two liquids A and B were being separated by distillation. After some time separation of components stopped and composition of vapour phase became same as that of liquid phase. Both the components started coming in the distillate. Explain why this happened.

Concept: Azeotrope formation

When two liquids form a non-ideal solution with significant deviations from Raoult's law, they can exhibit an extremum (maximum or minimum) in their boiling point–composition curve. At this special composition, the vapor phase has exactly the same composition as the liquid phase.

This composition is called an azeotrope (constant-boiling mixture). Because distillation relies on differences in volatility to separate components, and here the liquid and vapor have identical compositions, no further separation is possible — the mixture distills over unchanged. …

Why Separation Stopped: The Azeotrope Phenomenon

When you distill a binary mixture, you rely on the difference in volatility between the two components. The more volatile component (lower boiling point) preferentially enters the vapour phase, leaving the less volatile component behind in the liquid. This difference in composition between vapour and liquid is what drives separation.

But something remarkable happens at certain compositions: the vapour–liquid equilibrium curve crosses the diagonal on a temperature–composition diagram. At this special point, the composition of the vapour phase becomes identical to the composition of the liquid phase. This is called an azeotrope or azeotropic mixture.

What Happened During the Distillation

1. Initial stage: The mixture started with some composition where component A (say, the more volatile one) was enriched in the vapour phase. As distillation proceeded, A was removed preferentially, changing the liquid composition.

2. Approach to azeotrope: As the distillation continued, the liquid composition shifted along the boiling curve. The vapour composition also changed, but the gap between liquid and vapour compositions narrowed.

3. Azeotropic point reached: Eventually the liquid reached the azeotropic composition. At this point:

   • The vapour pressure contributions from both components balance in such a way that $y_A = x_A$ and $y_B = x_B$ (where $y$ denotes mole fraction in vapour, $x$ in liquid)
   • The mixture boils at a constant temperature like a pure substance
   • Both components have the same "effective volatility" at this composition

4. No further separation: Since vapour and liquid have identical compositions, distillation cannot separate them further. Both A and B distill over together in the same ratio they exist in the liquid.

The Two Types of Azeotropes

Concept: Azeotropes and Deviation from Raoult's Law

The phenomenon you described occurs because the mixture has formed a minimum-boiling azeotrope (or, less commonly, a maximum-boiling azeotrope).

Method: Identifying Azeotropic Behaviour

Step 1: Understand the condition for normal distillation

• For an ideal binary mixture (obeying Raoult's Law), the vapour phase is always richer in the more volatile component than the liquid phase.
• This allows separation by distillation — the more volatile component distils over first.

Step 2: Recognise the "stopping" condition

• When the vapour composition equals the liquid composition ($y_A = x_A$), the mixture boils at a constant temperature.
• This is the azeotropic point. At this composition, the mixture behaves like a pure substance — it cannot be separated further by simple distillation.

Step 3: Identify the cause — non-ideal behaviour

• Real liquid mixtures often show positive deviation from Raoult's Law (A–B interactions weaker than A–A and B–B interactions).
• This leads to a minimum in the boiling point-composition diagram.
• At the minimum boiling point, the vapour and liquid compositions are identical.

Step 4: Explain the consequence …

Here’s a clear, exam-focused breakdown of the common mistakes students make on this concept, along with how to avoid each.

The Core Concept (Why This Happens)

This situation describes the formation of an azeotropic mixture (specifically a minimum-boiling azeotrope). At a specific composition, the liquid and vapour phases have identical composition. Because of this, the mixture boils at a constant temperature, and simple distillation cannot separate the components further — both come over together in the distillate.

Common Mistake #1: Confusing Azeotropes with Simple Boiling

The Mistake:

Students think that “both components come in distillate” means the mixture is simply boiling vigorously, or that the distillation column is broken.

Why it’s wrong:

In normal boiling, the vapour is always richer in the more volatile component. Separation stops only when the vapour and liquid compositions become equal — that is the definition of an azeotrope.

How to Avoid:

• Remember: Azeotrope = constant boiling mixture where $\text{vapour composition} = \text{liquid composition}$.
• Draw the boiling point-composition diagram for a minimum-boiling azeotrope. Mark the point where the liquid and vapour curves touch — that’s the azeotropic point.

Common Mistake #2: Forgetting the “Why” — Intermolecular Forces

The Mistake:

Students give a vague answer like “because the mixture becomes azeotropic” without explaining why an azeotrope forms.

Why it’s wrong:

Examiners expect you to link the behaviour to deviation from Raoult’s law. A minimum-boiling azeotrope arises from positive deviation (weaker A–B interactions than A–A and B–B).

How to Avoid:

• State clearly:

  “Due to weaker intermolecular forces between A and B than between pure A and pure B, the mixture shows positive deviation from Raoult’s law. At a particular composition, the vapour pressure is maximum, so boiling point is minimum, and vapour composition equals liquid composition.”

• Example: Ethanol-water (95.6% ethanol, 78.2°C) — a classic exam case.

Common Mistake #3: Misidentifying the Type of Azeotrope

The Mistake:

Students say “it’s a maximum-boiling azeotrope” or just “azeotrope” without specifying.

Why it’s wrong:

The question says “after some time separation stopped” — this implies the mixture is boiling at a constant temperature lower than either pure component. That’s a minimum-boiling azeotrope.

How to Avoid:

• Learn the two types:
  • Minimum-boiling azeotrope (positive deviation) — e.g., ethanol-water, acetone-chloroform.
  • Maximum-boiling azeotrope (negative deviation) — e.g., nitric acid-water, phenol-aniline.
• In this question, the phrase “both components started coming in distillate” hints at a minimum-boiling type (since both are more volatile than the pure components at that composition).

Common Mistake #4: Not Mentioning the Constant Boiling Temperature

The Mistake:

Students describe the composition equality but forget to mention that the boiling point becomes constant.

Why it’s wrong:

The constant boiling temperature is the key experimental observation that confirms an azeotrope.

How to Avoid:

• Always add:

  “At this composition, the mixture boils at a constant temperature (lower than boiling points of pure A and B). Hence, distillation cannot separate them further.”

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