Acetaldehyde and benzaldehyde can be best distinguished by:

The correct answer is Option (4) → Fehling’s test

Fehling’s reagent is an alkaline solution of a \( \ce{Cu^2+} \) tartrate complex. On heating, an aliphatic aldehyde reduces it and a reddish-brown precipitate of Cu₂O forms; aromatic aldehydes do not respond to this test:

    \( \displaystyle \ce{CH3CHO + 2Cu^2+ + 5OH^- -> CH3COO^- + Cu2O v + 3H2O} \)

So acetaldehyde gives a positive test and benzaldehyde does not; Fehling’s test tells them apart.

• Option (1) 2,4 DNP test — Incorrect. Both compounds have a C=O group, and both give a coloured 2,4-dinitrophenylhydrazone.

• Option (2) Tollens’ test — Incorrect. Both aldehydes, aliphatic and aromatic, reduce Tollens’ reagent and give a silver mirror:

      \( \displaystyle \ce{RCHO + 2[Ag(NH3)2]^+ + 3OH^- -> RCOO^- + 2Ag v + 2H2O + 4NH3} \)

• Option (3) Sodium bicarbonate test — Incorrect. Effervescence of \( \ce{CO2} \) with \( \ce{NaHCO3} \) is given by carboxylic acids; neither aldehyde reacts.

NCERT (2026-27 edition):
Fehling’s Test – “Fehling reagent comprises of two solutions, Fehling solution A and Fehling solution B. Fehling solution A is aqueous copper sulphate and Fehling solution B is alkaline sodium potassium tartarate (Rochelle salt). These two solutions are mixed in equal amounts before test. On heating an aldehyde with Fehling’s reagent, a reddish brown precipitate is obtained. Aldehydes are oxidised to corresponding carboxylate anion. Aromatic aldehydes do not respond to this test.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: