Benedict's solution

This reagent is used to distinguish between aldehydes and ketones.

Benedict's solution contains the blue, aqueous copper(II) ion. This is a poor oxidizing agent. Ketones are not readily oxidized, but aldehydes oxidize very easily. Around a centimetre depth of Benedict's solution is placed in a boiling tube and a few drops of the suspected aldehyde are added. This mixture is then heated in a water bath. A positive test is given if an orange-red precipitate forms. The orange-red precipitate is copper(I) oxide, formed when the copper(II) ions were reduced on reaction with the aldehyde. The video below shows the reaction of Fehling's solution with glucose , a reducing sugar which contains an aldehyde group. Aldehydes, like propanal, give a less distinct change, but the brick red copper(I) oxide is still quite clear in the test tube after reaction.