Q.What is a reducing sugar?

Monosaccharides are the simplest carbohydrates: crystalline, sweet-tasting, water-soluble sugars that cannot be hydrolysed into any smaller carbohydrate. They follow the general formula $(CH_2O)_n$ (n = 3 to 7), giving the series triose, tetrose, pentose, hexose and heptose according to carbon count, and they are the monomer building blocks of all more complex carbohydrates.

A monosaccharide with a free aldehyde group (–CHO) is an aldose (e.g. glucose); one with a free ketone group is a ketose (e.g. fructose). Because of this free carbonyl group, every monosaccharide is a reducing sugar — capable of reducing Benedict's reagent (turning blue Cu²⁺ to a brick-red Cu⁺ precipitate), the basis of the standard test for a reducing sugar. …