Benedict's Test is used to detect Reducing Sugars.

• Reducing sugars have a free (or potentially free) aldehyde or ketone group that can reduce $C{u}^{2+}$ (blue complex) to $C{u}_{2}O$ (brick-red precipitate).
• Glucose, being an aldose, is a strong reducing sugar and gives a positive result.
• Sucrose is non-reducing because its functional groups are tied up in the glycosidic bond.
• Polysaccharides like Starch and Cellulose do not give a significant positive result because they lack a sufficient number of free hemiacetals.