The Benedict's test allows us to detect the presence of reducing sugars (sugars with a free aldehyde or ketone group).  All monosaccharides are reducing sugars; they all have a free reactive carbonyl group.  Some disaccharides have exposed carbonyl groups and are also reducing sugars.  Other disaccharides such as sucrose are non-reducing sugars and will not react with Benedict's solution.  Starches are also non-reducing sugars.  The copper sulfate (CuSO₄) present in Benedict's solution reacts with electrons from the aldehyde or ketone group of the reducing sugar to form cuprous oxide (Cu₂O), a red-brown precipitate.

CuSO_{4Cu⁺⁺ + SO4^--}
 

2 Cu⁺⁺  +  Reducing SugarCu+
        (electron donor)

Cu⁺Cu₂O (precipitate)

The final color of the solution depends on how much of this precipitate was formed, and therefore the color gives an indication of how much reducing sugar was present.


Increasing amounts of reducing sugar


green orange red brown
 

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