Which of the following statements are correct? A. Glycerol is purified by vacuum distillation because it decomposes at…

Step 1: Evaluate Statement A

"Glycerol is purified by vacuum distillation because it decomposes at its normal boiling point."

✅ CORRECT — Glycerol (propane-1,2,3-triol) has bp = 290°C but decomposes at that temperature. Distillation under reduced pressure lowers the effective bp (e.g., to ~150°C at ~15 mmHg), allowing purification without decomposition.

Step 2: Evaluate Statement B

"Aniline can be purified by steam distillation as aniline is miscible in water."

❌ INCORRECT — Aniline is immiscible with water (solubility: ~3.6 g/100 mL at 20°C). The requirement for steam distillation is that the compound be water-immiscible. Statement B gives the wrong reason — aniline qualifies for steam distillation precisely because it is immiscible, not miscible.

Step 3: Evaluate Statement C

"Ethanol can be separated from ethanol-water mixture by azeotropic distillation because it forms an azeotrope."

✅ CORRECT — Ethanol-water forms a minimum-boiling azeotrope at 95.6% ethanol (bp = 78.1°C). Normal fractional distillation cannot exceed 95.6% ethanol (called rectified spirit). Azeotropic distillation (adding benzene or cyclohexane) breaks this azeotrope to yield absolute alcohol (100% ethanol).

Step 4: Evaluate Statement D

"An organic compound is pure if mixed MP remains the same."

✅ CORRECT — The mixed melting point test: mixing the compound with a known pure sample. If the MP is unchanged → compound is pure (or identical to reference). Any impurity depresses the MP and broadens the melting range (eutectic mixture effect).

Correct Statements: A, C, D → Option (b)

Key Points to Remember:

• Aniline + water → steam distillation (aniline is water-immiscible!)
• Azeotrope: constant-boiling mixture that cannot be separated by normal distillation
• Mixed MP test: melting point depression indicates impurity
• Glycerol bp = 290°C → vacuum distillation; Glucose similarly uses vacuum distillation