Fehling's solution is a chemical reagent used to differentiate between water-soluble carbohydrate and ketone functional groups, and as a test for reducing sugars and non-reducing sugars, supplementary to the Tollens' reagent test.

What is the principle of Fehling test?

The principle of the Fehling test is based on the fact that the aldehyde group of sugar is oxidised by complexed copper ions to form acid. The red copper (I) oxide then precipitates, which is an indicator for the redox reaction. Sugars can exist in aqueous solution as a ring shape or as an open chain molecule.

How does Fehling solution work?

Fehling's solution reacts to all aldehydes (molecules with a -CH=O. group at one end) and to molecules that easily rearrange into aldehydes. The only sugars this includes are so-called "single sugars" or monosaccharides, such as glucose or fructose.