Which of the following shows amphoteric behaviour?

The correct answer is Option (1) → Glycine

• An amphoteric substance reacts with both acids and bases. Glycine, \( \ce{H2N-CH2-COOH} \), is an α-amino acid: it has an acidic carboxyl group and a basic amino group in the same molecule.

• In water the carboxyl group loses a proton and the amino group takes it up, giving the zwitter ion:

      \( \displaystyle \ce{H2N-CH2-COOH} \rightleftharpoons \ce{H3N^+-CH2-COO^-} \)

• The \( \ce{-COO^-} \) end takes up a proton from an acid and the \( \ce{-NH3^+} \) end gives a proton to a base, so glycine reacts with both.

Why the other options are incorrect

Option (2) Salicylic acid — Incorrect. It carries a carboxyl group and a phenolic –OH group, both acidic; it has no basic group to react with acids.

Option (3) Benzoic acid — Incorrect. Its only functional group is –COOH, so it behaves only as an acid.

Option (4) Aniline — Incorrect. It has only a basic amino group: amines are basic in nature and react with acids to form salts, but aniline has no acidic group to react with bases.

NCERT (2026-27 edition):
10.2.2 Classification of Amino Acids – “… In aqueous solution, the carboxyl group can lose a proton and amino group can accept a proton, giving rise to a dipolar ion known as zwitter ion.”

“In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases.”