MCQ Detailed View

Fehling’s solution is a chemical reagent used to test for the presence of reducing sugars and aldehydes. It consists of two solutions: Fehling’s A, which contains copper(II) sulfate, and Fehling’s B, which contains potassium sodium tartrate in a strong alkaline medium. When these two are mixed, they form a deep blue complex ion of copper(II) in alkaline solution.

Formaldehyde (HCHO) is the simplest aldehyde. Aldehydes are known as reducing agents because they can reduce the blue copper(II) ions (Cu²⁺) in Fehling’s solution to copper(I) oxide (Cu₂O). This reduction takes place under heating conditions. The copper(I) oxide that forms is insoluble and appears as a red precipitate.

The appearance of a brick-red precipitate is a positive test for aldehydes. In this case, HCHO reduces Fehling’s solution, and the characteristic red precipitate of Cu₂O confirms its aldehyde nature. This reaction can be written as:

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

This test is specific for aldehydes because ketones generally do not react with Fehling’s solution under normal conditions. That is why Fehling’s test is often used in the laboratory to distinguish between aldehydes and ketones. It is also applied in carbohydrate chemistry to test reducing sugars such as glucose, which give a similar positive red precipitate.

The other options represent incorrect outcomes. A white precipitate is not observed in this test. A yellow precipitate is linked with other reagents, such as iodine reaction with certain compounds. A blue color is the initial color of Fehling’s solution due to copper(II) ions, but it disappears after the reaction as it changes into the red precipitate of copper(I) oxide.

Thus, when Fehling’s solution reacts with HCHO, a red precipitate of Cu₂O is formed.