Fehling's reaction. Fehling's reagent is used to test for aldehydes. The reagent has a blue colour as it contains copper ions. The test tube in the middle contains Fehling's reagent that has been reduced by an aldehyde, to form a brown precipitate. The test tubes on the left and right contain Fehling's reagent and ketones. Ketones are very similar to aldehydes but do not react with Fehling's reagent, hence the colour is unchanged.