Q.What are reducing sugars ?

Reducing and Non-reducing Sugars

A reducing sugar is a carbohydrate that possesses a free aldehyde ($-CHO$) or a free ketone group — more precisely, a free anomeric carbon (the hemiacetal/hemiketal carbon that can open into an open-chain carbonyl form). Because this group can be oxidised, the sugar itself acts as a reducing agent and gives positive tests with Fehling's solution (brick-red $Cu_2O$ precipitate) and Tollens' reagent (silver mirror).

All monosaccharides — glucose, fructose, galactose — are reducing sugars. Interestingly, fructose, though a ketose, is also reducing because in alkaline medium it tautomerises to an aldose (glucose/mannose) and then reacts. Many disaccharides such as maltose and lactose are also reducing, since one of their two anomeric carbons remains free.

A non-reducing sugar has both anomeric carbons locked in the glycosidic linkage, leaving no free carbonyl. The classic example is sucrose:

$\text{Sucrose} \;\longrightarrow\; \underset{\text{glucose}}{C_6H_{12}O_6} \;+\; \underset{\text{fructose}}{C_6H_{12}O_6}$ …