MCQ Detailed View

Fehling's solution is a chemical reagent used to detect the presence of reducing aldehydes. It is an alkaline solution made from two parts: Fehling's A (aqueous copper(II) sulfate) and Fehling's B (a mixture of sodium potassium tartrate and sodium hydroxide). When combined, these form a deep blue complex of Cu²⁺ ions stabilized by tartrate ions in basic medium.

Formaldehyde (HCHO) is the simplest aldehyde and acts as a strong reducing agent. When heated with Fehling’s solution, it reduces the blue copper(II) ions (Cu²⁺) to copper(I) oxide (Cu₂O), which is a red precipitate.

The balanced redox reaction is:

HCHO + 2Cu²⁺ + 5OH⁻ → HCOO⁻ + Cu₂O (red ppt) + 3H₂O

In this reaction, the aldehyde group (–CHO) of formaldehyde is oxidized to a carboxylate ion (HCOO⁻), while the Cu²⁺ ions are reduced to Cu⁺, which then precipitates as red-colored Cu₂O.

This test is specific for aldehydes and is not given by most ketones. The appearance of a red precipitate confirms the presence of an aliphatic aldehyde like formaldehyde. Fehling’s solution remains blue if no aldehyde is present or if the compound is not a reducing agent.

The red precipitate formed is characteristic of positive Fehling's test. Yellow or white precipitates are not associated with this reaction. The original color of the Fehling solution is blue due to Cu²⁺, but this disappears as the red Cu₂O precipitate forms.