All monosaccharides are known to be reducing sugars. Glucose, being an aldose, features a free aldehyde group in its open-chain form. Moreover, it can be categorized as reducing sugar.
Reducing sugar is one that can act as a reducing agent. It can reduce other compounds but oxidize itself because it has a free aldehyde group or a free ketone group.
The free aldehyde group is easily oxidized to carboxylic acids. So, it can oxidize itself but not others.
