Q.Why is it not possible to obtain pure ethanol by fractional distillation? What general name is given to binary mixtures which show deviation from Raoult's law and whose components cannot be separated by fractional distillation. How many types of such mixtures are there?

The key idea is that ethanol and water form a minimum-boiling azeotrope — a mixture that boils at a constant composition and cannot be separated by simple fractional distillation.

Reasoning:

1. Ethanol and water show a large positive deviation from Raoult’s law. The intermolecular forces between ethanol and water are weaker than those in the pure components, so the total vapour pressure of the mixture is higher than predicted.
2. This results in a boiling point–composition diagram with a minimum boiling point at a specific composition (about 95.6% ethanol by volume at 1 atm). At this point, the vapour has the same composition as the liquid.
3. When you distil a mixture with less than 95.6% ethanol, the vapour is richer in ethanol, so the liquid left behind becomes richer in water. Once the liquid reaches the azeotropic composition, both liquid and vapour have the same composition — further distillation yields no change. …

Why fractional distillation fails for ethanol

Fractional distillation works because different liquids have different boiling points. When you heat a mixture, the vapour is richer in the more volatile component. By repeatedly condensing and re-vaporising, you can separate the components almost completely — provided the vapour composition keeps shifting away from the liquid composition.

Ethanol (boiling point 78.4 °C) is more volatile than water (100 °C). So you’d expect that distilling a dilute ethanol solution would give you progressively stronger ethanol. And it does — up to a point.

But at about 95.6% ethanol by mass (roughly 96% by volume), something strange happens: the vapour has exactly the same composition as the liquid. The mixture boils at a constant temperature of 78.2 °C — lower than either pure ethanol or pure water. This is a minimum-boiling azeotrope.

Once you reach this composition, further distillation does nothing. The liquid and vapour are identical in composition, so no enrichment occurs. You cannot cross this barrier by simple or fractional distillation. The mixture behaves as if it were a single pure substance.

The general name: azeotropes

Binary mixtures that deviate from Raoult’s law so strongly that their vapour and liquid compositions become equal at some point are called azeotropes (from Greek: “to boil unchanged”). They cannot be separated into pure components by fractional distillation alone.

Two types of azeotropes

There are exactly two types of azeotropic mixtures:

1. Minimum-boiling azeotropes — The mixture boils at a temperature lower than either pure component. Ethanol–water is the classic example. These arise from positive deviations from Raoult’s law (the A–B interactions are weaker than A–A and B–B interactions, so the mixture is “easier” to boil). …

Concept: Non-Ideal Solutions & Azeotropes

The relevant concept is deviation from Raoult's law in liquid-liquid binary mixtures, leading to the formation of azeotropes (constant-boiling mixtures).

Method: Identifying and Classifying Azeotropes

Method name: Azeotrope Analysis (based on deviation from Raoult's law)

Steps:

1. Check for deviation from Raoult's law

   • If the mixture shows positive deviation (vapour pressure > ideal), the intermolecular forces (A–B) are weaker than pure components (A–A and B–B).
   • If the mixture shows negative deviation (vapour pressure < ideal), the A–B forces are stronger.

2. Identify the azeotrope

   • An azeotrope is a mixture that boils at a constant temperature and has the same composition in liquid and vapour phases.
   • Because of this, fractional distillation cannot separate the components beyond the azeotropic composition.

3. Classify the azeotrope

   • Minimum boiling azeotrope — occurs with positive deviation (e.g., ethanol–water).
   • Maximum boiling azeotrope — occurs with negative deviation (e.g., nitric acid–water).

Answer to the Questions

Why is pure ethanol not obtained by fractional distillation?

• Ethanol and water form a minimum boiling azeotrope at 95.6% ethanol (by volume) and 78.15 °C.
• At this composition, the vapour has the same composition as the liquid.
• Further distillation cannot increase ethanol concentration beyond this point — the azeotrope acts as a "barrier."

General name for such binary mixtures? …

Here is a breakdown of the common mistakes students make on this specific concept, along with the correct reasoning and exam-ready answers.

The Core Concept: Azeotropes

First, let's establish the correct answer to the question, as understanding this is the only way to avoid mistakes.

• Why can't we get pure ethanol by fractional distillation?

  Ethanol and water form a minimum-boiling azeotrope. At a specific composition (approximately 95.6% ethanol, 4.4% water by volume), the mixture boils at a constant temperature (78.15°C), which is lower than the boiling point of pure ethanol (78.37°C) and pure water (100°C).

  When you distill a dilute ethanol solution, the vapor is richer in ethanol, but once the liquid in the flask reaches the azeotropic composition, the vapor has the same composition as the liquid. Further distillation yields only the azeotrope, not pure ethanol.

• General name for such mixtures: Azeotropes (or azeotropic mixtures).

• How many types? Two types:

  1. Minimum-boiling azeotropes (positive deviation from Raoult's law) — e.g., ethanol-water.
  2. Maximum-boiling azeotropes (negative deviation from Raoult's law) — e.g., nitric acid-water.

Common Mistakes & How to Avoid Them

Mistake 1: Saying "Pure ethanol cannot be obtained because it forms a constant boiling mixture" without explaining why it boils constantly.

• Why it's wrong: This is a memorized phrase without understanding. Examiners want to see that you know the mechanism: at the azeotropic composition, the vapor and liquid phases have identical composition, so separation by simple or fractional distillation stops.

• How to avoid: Always link the constant boiling point to equal composition of vapor and liquid. Write:

  "At the azeotropic composition, the mole fraction of ethanol in the vapor equals that in the liquid. Hence, no further enrichment occurs."

Mistake 2: Confusing "fractional distillation" with "simple distillation" in the context of azeotropes.

• Why it's wrong: Some students think that using a fractionating column (fractional distillation) can separate azeotropes. It cannot. Fractional distillation is more efficient for separating close-boiling liquids, but it fails when the vapor and liquid compositions become identical.

• How to avoid: Remember: No distillation method (simple or fractional) can separate an azeotrope into its pure components. The only way is to break the azeotrope using a different technique (e.g., adding benzene to form a ternary azeotrope, or using molecular sieves).

Mistake 3: Forgetting to mention that ethanol-water is a minimum-boiling azeotrope.

• Why it's wrong: The question asks "Why is it not possible?" — the answer must specify the type of deviation. If you just say "it shows deviation from Raoult's law," you lose marks.

• How to avoid: Always classify the azeotrope:

  "Ethanol-water shows positive deviation from Raoult's law, forming a minimum-boiling azeotrope."

Mistake 4: Stating that there are "three types" of azeotropes (e.g., positive, negative, and ideal).

• Why it's wrong: There is no "ideal azeotrope." An ideal solution obeys Raoult's law exactly and can be separated by fractional distillation. Azeotropes are defined by their deviation from Raoult's law, which gives exactly two types: minimum-boiling (positive deviation) and maximum-boiling (negative deviation).

• How to avoid: Memorize the two types with one example each:

  • Minimum-boiling: Ethanol-water, isopropanol-water.
  • Maximum-boiling: Nitric acid-water, hydrochloric acid-water (at constant pressure). …