Q.[Note: two or more options may be correct] Which of the following binary mixtures will have same composition in liquid and vapour phase?

Concept: Azeotropes – constant-boiling mixtures

A binary liquid mixture has the same composition in liquid and vapour phases when it forms an azeotrope. At the azeotropic composition, the system behaves as if it were a pure component: distillation cannot separate the components further because $x_i = y_i$ for both.

Reasoning:

1. Ideal solutions (Benzene–Toluene, n-Hexane–n-Heptane) obey Raoult's law closely and show no azeotrope. Their vapour is always richer in the more volatile component, so $x \neq y$ at all compositions. …

When a liquid mixture boils, the vapour is usually richer in the more volatile component—that's the basis of distillation. But certain mixtures reach a special composition where the liquid and vapour have exactly the same makeup. At this point, called an azeotrope, the mixture behaves like a pure substance: it boils at a constant temperature and the vapour has the same mole fractions as the liquid. No amount of simple distillation can separate an azeotropic mixture further.

Azeotropes arise from strong intermolecular interactions between unlike molecules. When these interactions are significantly different from those in the pure components, Raoult's law breaks down and the vapour pressure curve develops an extremum (maximum or minimum). At that extremum, the composition of liquid and vapour coincide.

Now let's examine each pair:

1. Benzene–Toluene (i)

   Both are non-polar aromatic hydrocarbons with very similar structures and intermolecular forces (London dispersion). They form an ideal solution that obeys Raoult's law closely across all compositions. The vapour is always enriched in benzene (the more volatile component), so liquid and vapour compositions differ at every point. No azeotrope forms.

2. Water–Nitric acid (ii)

   This system exhibits strong hydrogen bonding and significant deviations from ideality. Water and nitric acid form a maximum-boiling azeotrope at approximately 68% (by mass) $\ce{HNO3}$ and 120.5 °C. At this composition, liquid and vapour have the same concentration.

3. Water–Ethanol (iii)

   Ethanol and water hydrogen-bond with each other, but the ethanol–water interaction is weaker than water–water hydrogen bonds. This leads to positive deviation from Raoult's law and a minimum-boiling azeotrope at roughly 95.6% ethanol (by mass) and 78.2 °C. At the azeotropic point, both phases share the same composition—this is why you cannot distill absolute (100%) ethanol from a water–ethanol mixture by simple distillation. …

Concept: Azeotropes

An azeotrope is a mixture of two liquids that boils at a constant temperature and has the same composition in the liquid and vapour phase. This happens when the mixture's boiling point is either a minimum or a maximum on the boiling point-composition diagram.

Method: Identify Azeotrope-Forming Mixtures

Step 1: Recall the types of binary mixtures

• Ideal mixtures (Raoult's law obeyed): No azeotrope. Composition in liquid and vapour is always different (except at pure components).
• Non-ideal mixtures with large deviations: Can form azeotropes.

Step 2: Check each option

Common Mistakes & How to Avoid Them

Mistake 1: Assuming all ideal solutions have same composition in both phases

Why it's wrong:

Only azeotropic mixtures have identical liquid and vapour composition at a specific boiling point. Ideal solutions (like Benzene-Toluene, n-Hexane-n-Heptane) follow Raoult's law perfectly — their vapour is always richer in the more volatile component.

How to avoid:

Remember:

• Ideal solutions → vapour composition ≠ liquid composition (except at pure component ends)
• Azeotropes → at a specific composition, vapour = liquid composition

Mistake 2: Confusing minimum-boiling azeotropes with maximum-boiling ones

Why it's wrong:

Students often think all non-ideal solutions form azeotropes, but they forget the type:

• Water-Ethanol → minimum-boiling azeotrope (vapour richer in ethanol below azeotropic composition)
• Water-Nitric acid → maximum-boiling azeotrope (vapour richer in water below azeotropic composition)

Both have a point where composition is identical — but students miss that both are correct answers here.

How to avoid:

Learn the classic examples:

Mistake 3: Thinking "same composition" means at all compositions

Why it's wrong:

The question asks: "will have same composition in liquid and vapour phase" — this happens only at the azeotropic point, not for all mixtures of those components.

How to avoid:

Read carefully: "will have" means there exists a composition where this is true. For azeotropic mixtures, yes. For ideal solutions, no such composition exists (except pure components, which aren't mixtures).

Mistake 4: Selecting only one option when multiple are correct

Why it's wrong:

The question explicitly says "[Note: two or more options may be correct]" — yet students often pick just (iii) Water-Ethanol, forgetting (ii) Water-Nitric acid.

How to avoid:

• Always check the note at the top
• For azeotropes: both minimum-boiling AND maximum-boiling azeotropes satisfy the condition
• Memorise:
  • Minimum-boiling azeotropes: Water-Ethanol, Water-Isopropanol, Acetone-Chloroform …