The only carboxylic acid which gives positive Tollens' test is
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Formic acid
Tollens’ test is given by compounds that are easily oxidised, typically aldehydes, which reduce ammoniacal silver nitrate to a silver mirror. Formic acid (HCOOH) is the only carboxylic acid with a hydrogen on the carbonyl carbon, so its molecule contains an aldehyde-like H–C=O group. It is oxidised to \( \ce{CO2} \) and water and deposits silver:
\( \ce{HCOOH + 2[Ag(NH3)2]+ + 2OH- -> 2Ag v + CO2 + 2H2O + 4NH3} \)
Why the other options are incorrect
Option (1) Acetic acid — Incorrect. \( \ce{CH3COOH} \) has no H on the carbonyl carbon and is not oxidised by Tollens’ reagent.
Option (3) Oxalic acid — Incorrect. It is oxidised by strong oxidants such as acidified \( \ce{KMnO4} \), but it has no aldehyde H and does not give the silver mirror.
Option (4) Phthalic acid — Incorrect. An aromatic dicarboxylic acid with no aldehyde group; it does not reduce Tollens’ reagent.
NCERT (2026-27 edition):
Oxidation
