Aldoses and ketoses are reducing sugars. This means that, when heated with an alkaline solution of copper (II) sulphate (a blue solution called benedict’s solution), the aldehyde or ketone group reduces Cu2+ ions to Cu+ ions forming a brick-red precipitate of copper (I) oxide. In the process, the aldehyde or ketone group is oxidised to a carboxyl group (- COOH). This reaction is used as a test for reducing sugar and is known as Benedict’s test. The results of benedict’s test depend on the concentration of the sugar. If there is no reducing sugar it remains blue.
- Sucrose is not a reducing sugar.
- The greater the concentration of reducing sugar, the more is the precipitate formed and greater is the colour change.
