Benedict’s test is similar to Fehling’s test, but, uses a single solution of copper sulphate in aqueous sodium hydroxide with citrate instead of tartrate. copper(II) citrate does not deteriorate as quickly on standing. Reducing sugars reduce soluble bluer copper sulphate, containing copper(II) ions to an insoluble red-brown copper oxide containing copper(I). To give a positive test, the carbohydrate must contain a hemiacetal which will hydrolyse in aqueous solution to the aldehyde form.
The colour ranges from green to yellow to orange to brick-red depending on the amount of reducing sugar in the sample; with a sample containing 1% glucose, the precipitate is usually brick-red. Benedicts’s test will give a colour change for any mono- or disaccharide containing a hemiacetal or hemiketal group. Since sucrose or table sugar does not contain these groups, it will not give a positive test.