Assertion (A): It is not possible to separate the components of an azeotrope by fractional distillation.
Reason (R): Components of an azeotrope have the same composition in liquid and vapour phase and boil at a constant temperature.
Assertion (A): It is not possible to separate the components of an azeotrope by fractional distillation.
Reason (R): Components of an azeotrope have the same composition in liquid and vapour phase and boil at a constant temperature.
Options
Correct option: (A) — Both A and R are true, and R correctly explains A.
Why A is true:
- Fractional distillation separates liquids by exploiting a difference in composition between the boiling liquid and the vapour above it.
- An azeotrope boils without this difference, so distillation cannot enrich either component.
Why R is the correct explanation:
- At the azeotropic composition, the liquid and vapour have identical composition and the mixture boils at a constant temperature (like a pure liquid).
- Because the vapour is not richer in either component, repeated distillation stages produce no separation — this is exactly the reason stated in A.
Marking Scheme
- 11 mark: correct code (A) — both statements true and Reason is the correct explanation of the Assertion.
- 2No partial credit for a code alone without matching the standard assertion–reason logic; full mark awarded for (A).
Hint
Ask whether liquid and vapour compositions differ — if they are equal, distillation cannot separate.
Quick Oral Answer
Both statements are true and the reason explains the assertion: an azeotrope has identical liquid and vapour compositions and boils at a fixed temperature, so fractional distillation cannot separate its components.
Analysis & Explanation
Concept:
Azeotropes are constant-boiling binary mixtures that distil unchanged in composition. They arise at compositions where a solution shows either maximum or minimum in its vapour-pressure/boiling-point curve.
Two types:
- Minimum boiling azeotropes — formed by solutions showing large positive deviation from Raoult's law (e.g. ethanol–water, ~95% ethanol).
- Maximum boiling azeotropes — formed by solutions showing large negative deviation (e.g. nitric acid–water, ~68% HNO₃).
Exam trap:
Students often mark (D) thinking the assertion is false because "anything can be separated." In reality, at the azeotropic point the liquid and vapour compositions are equal, so fractional distillation genuinely fails — the assertion is true and the reason is its exact cause.
Real-world:
Breaking the ethanol–water azeotrope to make anhydrous (absolute) alcohol requires special techniques such as azeotropic distillation with benzene or the use of molecular sieves, not ordinary fractional distillation.
Common Mistakes
- 1Marking (B) — treating the reason as unrelated, when it is precisely why the components cannot be separated.
- 2Confusing azeotropes with ideal solutions; ideal solutions have no azeotrope and can be separated by distillation.
- 3Assuming absolute alcohol is obtained by simple fractional distillation of the ethanol–water azeotrope.
Interesting Facts
The ethanol–water azeotrope boils at 78.1 °C and fixes at about 95.6% ethanol by mass — the reason 'rectified spirit' cannot exceed this purity by ordinary distillation.
Hydrochloric acid forms a maximum-boiling azeotrope at about 20.2% HCl boiling near 110 °C, which is why constant-boiling HCl is used as a primary standard in volumetric analysis.
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Frequently Asked Questions
Why can't an azeotrope be separated by fractional distillation?
Because at the azeotropic composition the vapour has exactly the same composition as the boiling liquid. Fractional distillation works only when the vapour is richer in one component; with no compositional difference, no number of distillation stages can enrich or separate the components.
What is the difference between minimum and maximum boiling azeotropes?
Minimum boiling azeotropes form from solutions with large positive deviation from Raoult's law (weaker A–B interactions), e.g. ethanol–water. Maximum boiling azeotropes form from solutions with large negative deviation (stronger A–B interactions), e.g. nitric acid–water.