Read the passage carefully and answer the questions.
Carbohydrates such as cane sugar, glucose, starch etc. belong to a class of naturally occurring organic compounds. Structurally these are polyhydroxy aldehydes or ketones or the compounds which produce such units on hydrolysis. Carbohydrates which are sweet in taste are called sugars. Theses sugars can be reducing or non-reducing. Monosaccharides cannot be hydrolyzed to simpler units whereas Oligosaccharides yield two to ten monosaccharide units on hydrolysis. Polysaccharides yield large number of monosaccharide units on hydrolysis. This class of biomolecules is essential for living systems as they serve as storage molecules in plants and animals. In addition, they provide raw materials for important industries like furniture, textiles, paper, lacquers and breweries. Two aldopentose form important constituents of nucleic acids. Carbohydrates are also called saccharides (Greek: sakcharon which means sugar). Sugar which is used in household and in milk also comes under this class of biomolecules.
Reducing sugars respond to
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Both Tollen's and Fehling's reagent
A reducing sugar is one with a free aldehyde group, or a ketone group that can give one, so that it can be oxidised by mild oxidising agents. All monosaccharides, aldoses and ketoses alike, are reducing sugars, and so are disaccharides such as maltose and lactose; sucrose is not, because both of its reducing groups are used in the glycosidic bond.
A reducing sugar gives both standard tests:
With Tollens’ reagent (ammoniacal silver nitrate) it gives a silver mirror: \( \ce{Ag+} \) is reduced to Ag.
With Fehling’s solution it gives a reddish brown precipitate: \( \ce{Cu^2+} \) is reduced to \( \ce{Cu2O} \).
So reducing sugars respond to both Tollens’ and Fehling’s reagent.
Why the other options are incorrect
Option (1) Tollens' reagent — Incorrect. True, but incomplete: reducing sugars respond to Fehling’s solution as well.
Option (2) Fehling's Reagent — Incorrect. True, but incomplete: reducing sugars respond to Tollens’ reagent as well.
Option (4) Schiff's Reagent — Incorrect. Schiff’s reagent detects a free aldehyde group, but glucose does not give the Schiff’s test, because it exists mainly in the cyclic form; this was one of the facts that showed glucose has a ring structure.
NCERT (2026-27 edition):
Classification of Carbohydrates – …
