Which one of the following statements about Fehling’s test is not correct?
[amp_mcq option1=”Fehling’s A solution is aqueous copper sulphate.” option2=”Fructose gives a positive Fehling’s test.” option3=”Red-brown ppt of CuO is obtained in the reaction.” option4=”Aromatic aldehydes do not respond to this test.” correct=”option3″]
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UPSC Geoscientist – 2023
Fehling’s test is a qualitative test used to distinguish between reducing and non-reducing sugars, and also to detect aldehydes. It involves Fehling’s solution, which is prepared fresh by mixing Fehling’s A (aqueous copper(II) sulfate) and Fehling’s B (aqueous sodium potassium tartrate and a strong alkali like NaOH or KOH). Reducing sugars (like glucose, fructose – due to isomerization) and aldehydes reduce the blue copper(II) ions (Cu²⁺) in Fehling’s solution to red-brown copper(I) oxide (Cu₂O) precipitate. Statement C claims the precipitate is red-brown CuO (copper(II) oxide), which is black. The precipitate formed is red-brown Cu₂O (copper(I) oxide). Therefore, statement C is incorrect.
Fehling’s test involves the reduction of Cu²⁺ ions to Cu⁺ ions, forming a red-brown precipitate of Cu₂O.