The organic compound is treated with Fehling's solution and warmed over a water bath. Appearance of a red or orange precipitate confirms the presence of aldehyde group.
The aldehyde reduces the Cu2+ (cupric ions) to cuprous state (Cu2O cuprous oxide).
Tollen's Reagent Test
To the organic compound, an equal amount of Tollen's reagent is added and the mixture is warmed on a water bath. Appearance of shining silver mirror on the inner walls of the test tube confirms the presence of aldehydic group.
The Tollen's reagent (ammonical silver nitrate solution) contains silver ions. These are reduced by aldehydes to metallic silver.