Azeotropes and Distillation
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Direct answer
Ethanol stops at 95.6 per cent: past that composition, the liquid boils at a constant temperature (78.2 degrees C, below either pure component) and the vapour carries exactly the same composition as the liquid, so no amount of fractionation can enrich it further — that is a minimum-boiling azeotrope, born of positive deviation from Raoult's law. Negative deviation, where unlike molecules attract more strongly than like ones, produces the mirror image: maximum-boiling azeotropes such as 68 per cent nitric acid with water (about 393.5 K) and 20.2 per cent hydrogen chloride with water. Ideal solutions obeying Raoult's law across all proportions (benzene-toluene) never azeotrope, and breaking one demands a third component or a pressure change, never another plate in the column.
What you must remember
- Azeotrope definition: a binary mixture that boils at constant temperature with vapour and liquid of identical composition, behaving like a single substance to the distillation column — separation by fractionation fails by definition.
- Positive deviation signature: weaker A-B interactions than A-A and B-B, higher vapour pressure than Raoult predicts, heat absorbed on mixing, and a minimum-boiling azeotrope — ethanol-water at 95.6 per cent ethanol by mass, 78.2 degrees C; acetone-carbon disulphide is the standard second example.
- Negative deviation signature: stronger A-B attraction (often hydrogen bonding between the unlike partners), lower vapour pressure, heat released on mixing, and a maximum-boiling azeotrope — nitric acid-water and hydrogen chloride-water; acetone-chloroform is the classic H-bonded example.
- Column mechanics: vapour is always richer in the more volatile component until the azeotropic composition, where the relative volatility collapses to one and enrichment dies.
- Breaking tools: azeotropic distillation with an entrainer (benzene or cyclohexane historically added to ethanol-water to carry water off as a ternary mixture), pressure-swing distillation exploiting the composition's pressure dependence, and chemical drying with quicklime or molecular sieves for absolute alcohol.
- Raoult's law baseline: p_total = x_A p_A° + x_B p_B° holds across all proportions only for ideal pairs with delta H mixing = 0 and delta V mixing = 0.
- Why water-ethanol azeotropes matter industrially: rectified spirit caps near 95.6 per cent; absolute alcohol requires the extra drying step, a one-line question JEE has repeatedly asked.
- Colligative contrast: azeotropes are vapour-pressure phenomena, not colligative ones — no mole-counting shortcuts apply.
Why the distillate composition freezes
Follow a 10 per cent ethanol fermentation broth up a fractionating column. Each theoretical plate equilibrates liquid with vapour, and each time the vapour grows richer in ethanol, the more volatile player; by mid-column the composition passes 80 per cent, then 90 per cent, then closes on 95.6 per cent — and there the mathematics of enrichment die, because at the azeotropic composition the vapour's composition equals the liquid's. With no composition gap between the phases, no plate, however perfect, can push further; the distillate arrives at exactly 95.6 per cent and the pot residue trends toward pure water. The same reasoning inverted explains maximum-boiling mixtures: distil dilute nitric acid and the distillate is mostly water, the residue climbing until it locks at the 68 per cent azeotrope — the strongest acid a simple still can concentrate from dilute feed.
How the exam frames it
Solutions, including ideal and non-ideal behaviour, remain a listed JEE Main unit, and the exam's favourite moves are pairing a named mixture with its deviation sign and its azeotrope type, or asking what fractionation cannot achieve; JEE Advanced adds vapour-composition diagram reasoning. The traps are durable ones: coupling positive deviation with maximum boiling (it is minimum — higher vapour pressure means lower boiling), quoting the ethanol azeotrope in mole per cent (the figure is by mass), and proposing "better fractionation" as the cure for an azeotrope. Note that the Raoult's-law deviations also decide the sign of enthalpy and volume of mixing — a pairing the examiners recycle.
Frequently asked questions
Why can fractional distillation not cross 95.6 per cent ethanol?
At the azeotropic composition, vapour and liquid have identical composition, so there is no composition difference left for any number of plates to exploit.
What causes positive deviation from Raoult's law?
A-B interactions weaker than the like-molecule interactions, so molecules escape the surface more easily than ideality predicts, raising vapour pressure above the ideal line.
Name two maximum-boiling azeotropes with their compositions.
Nitric acid-water at 68 per cent HNO3 (about 393.5 K) and hydrogen chloride-water at 20.2 per cent HCl, both from negative deviations driven by strong unlike-molecule attraction.
How is absolute alcohol prepared from rectified spirit?
By drying the 95.6 per cent azeotrope with quicklime (or molecular sieves) and then distilling, or by azeotropic distillation with an entrainer such as cyclohexane that carries the water off.
What is the vapour composition above an azeotropic liquid?
Identical to the liquid's composition — that equality is the defining property that makes the mixture distil unchanged.