All monosaccharides contain the ability to undergo oxidation from a weak oxidizing agent and are therefore reducing sugars. Benedict's solution is one test commonly used to check for the presence of reducing sugars.
Monossaccharides are the most basic carbohydrate. They consist of a single sugar unit. In closed-chain form, the C1 hydroxyl group is easily oxidized due to that carbon's close proximity to an oxygen when in the ring form. Some disaccharides are reducing dependent on the way in which the sugar units have combined.
Oxidation is the transfer of electrons from one molecule, in this case a sugar, to another element. The molecule that receives the electrons is said to undergo a reduction. In a Benedict's solution test, the sugar in question is combined with Benedict's reagent. The reagent is an aqueous solution composed of copper (II) sulphate, sodium carbonate and sodium citrate. The mixture is heated to 95 degrees Celsius and stirred. When copper (II) sulphate is reduced, it releases a reddish powder. If the test yields a reddish-orange precipitate, the sugar in question is considered reducing. If the test remains a pale blue, the test result is considered negative, and no reducing sugars are present. Tollen's and Fehling's reagents can also be used to test for the presence of reducing sugars.