Absolute alcohol cannot be obtained by simple fractional distillation because
(A) pure ethyl alcohol is unstable
(B) ethyl alcohol forms H-bonding with water
(C) constant boiling azeotropic mixture is formed with water
(D) boiling point of ethyl alcohol is less than that of water
The correct answer is: (C) constant boiling azeotropic mixture is formed with water
Explanation:
- Absolute alcohol is 100% pure ethanol (no water).
- Ethanol and water form an azeotropic mixture at approximately 95.6% ethanol and 4.4% water, which boils at 78.1°C (lower than the boiling point of either pure ethanol or pure water).
Because of this:
- When a mixture is distilled, the vapor has the same composition as the liquid at the azeotropic point.
- Thus, no further separation is possible by simple fractional distillation beyond this point.
Why the other options are incorrect:
- (A): Ethanol is a stable compound.
- (B): Ethanol does form hydrogen bonds with water, but this is not the main reason fractional distillation fails.
- (D): The boiling point difference matters but doesn’t prevent separation—it’s the azeotrope that does.
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