MODEL RELEASED. Fehling's test for sugars. Student heating a test tube containing glucose (a simple sugar) and Fehling's reagent. The brown colour is a positive result for the presence of an aldehyde group, a chemical structure found in sugars. The Fehling's reagent contains copper ions, giving it its blue colour. The copper ions oxidize the sugar and are themselves reduced to a lower oxidation state in copper oxide (brown). This change in the oxidation state is what causes this colour change. Fehling's reagent is a mixture of the two bottled reagents seen: A (copper sulphate solution, blue) and B (alkaline potassium sodium tartrate, clear).