Fehling's Solution (a mixture of Copper sulfate and alkaline Sodium potassium tartrate) is a mild oxidizing agent.

• Aliphatic Aldehydes: Readily reduce the deep blue $C{u}^{2+}$ complex to a reddish-brown precipitate of Cuprous oxide ($C{u}_{2}O$).
• Ketones: Generally do not react because they lack the necessary $H$ atom attached to the carbonyl carbon for easy oxidation.
• Aromatic Aldehydes (e.g., Benzaldehyde) are also a classic exception and do not reduce Fehling's solution, making it a useful test to distinguish them from aliphatic ones.