In organic chemistry, Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone (>C=O) functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' reagent test.

However, the coordination chemistry is complex and various species with different metal to ligand ratio have been determined.

[2][3][4][5][6] Other methods of preparing comparable cupric-ion test-reagent solutions were developed at about the same time as Fehling's.

It will give a positive result for aldose monosaccharides (due to the oxidisable aldehyde group) but also for ketose monosaccharides, as they are converted to aldoses by the base in the reagent, and then give a positive result.

The net reaction between an aldehyde and the copper(II) ions in Fehling's solution may be written as: or with the tartrate included: