JEE Main 2021 · 27 July · morning · Single correct · +4 / −1
Which one among the following chemical tests is used to distinguish monosaccharide from disaccharide?
- (a) Seliwanoff's test
- (b) Iodine test
- (c) Barfoed test
- (d) Tollen's test
Answer and solution
Answer (c) Barfoed test
Solution
Chemistry ma’am · 3 steps
Barfoed's test works like a stopwatch: monosaccharides reduce copper fast, disaccharides lag far behind.
- Barfoed's reagent. It is copper(II) acetate, Cu(CH₃COO)₂, dissolved in dilute acetic acid. A sugar that reduces Cu²⁺ gives a brick-red precipitate of Cu₂O.
- Why timing matters. The medium is weakly acidic, so only strong reducing sugars react quickly. Monosaccharides give the red precipitate within about 2 to 3 minutes of heating. Reducing disaccharides such as maltose and lactose react much more slowly, often only after about 10 minutes, or not at all.
Test Reagent What it tells apart Seliwanoff's resorcinol in conc. HCl ketoses (fast cherry red) from aldoses Iodine I₂ in KI starch (blue-black) from other carbohydrates Barfoed's Cu(CH₃COO)₂ in acetic acid monosaccharides (fast) from disaccharides (slow) Tollens' ammoniacal AgNO₃ reducing from non-reducing sugars - So. Only Barfoed's test separates a monosaccharide from a disaccharide, so the answer is (c).
The idea to keep. Match each sugar test to the one difference it detects: Seliwanoff for ketose vs aldose, iodine for starch, Tollens for reducing sugars, Barfoed for monosaccharide vs disaccharide.
Answer from the NTA final answer key. Solution by PrepWiser.