Fehling solution B is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → alkaline sodium potassium tartrate.
Fehling solution B is alkaline sodium potassium tartrate (Rochelle salt); solution A is aqueous copper sulphate. The two are kept apart and mixed only before the test, because copper(II) in alkali alone would precipitate as copper(II) hydroxide. The tartrate ions of B form a soluble complex with \( \ce{Cu^2+} \), which keeps the copper in solution in the alkaline medium.
On heating an aldehyde with Fehling reagent, the aldehyde is oxidised to the carboxylate ion and \( \ce{Cu^2+} \) is reduced to a reddish brown precipitate of copper(I) oxide:
\( \displaystyle \ce{R-CHO + 2Cu^2+ + 5OH^- -> R-COO^- + Cu2O v + 3H2O} \)
NCERT (2026-27 edition):
Oxidation – …
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| 20-05 S2 | 22-05 S1 | – | – | – |