Amino acids contain amino (–NH2) and carboxyl (–COOH) functional groups. Depending upon the relative position of amino group with respect to carboxyl group, the amino acids can be classified as α, β, γ, δ and so on. Only α-amino acids are obtained on hydrolysis of proteins. All α-amino acids have trivial names, which usually reflect the property of that compound or its source. Amino acids are classified as acidic, basic or neutral depending upon the relative number of amino and carboxyl groups in their molecule. Equal number of amino and carboxyl groups makes it neutral; more number of amino than carboxyl groups makes it basic and more carboxyl groups as compared to amino groups makes it acidic. The amino acids, which can be synthesized in the body, are known as nonessential amino acids. On the other hand, those which cannot be synthesized in the body and must be obtained through diet, are known as essential amino acids.
The acid showing salt-like character in aqueous solution is
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → α-Aminoacetic acid
α-Aminoacetic acid is glycine, \( \ce{H2N-CH2-COOH} \). It has an acidic group (–COOH) and a basic group (–NH2) in the same molecule. In aqueous solution the carboxyl group loses a proton and the amino group takes it up, giving a zwitter ion (dipolar ion):
\( \displaystyle \ce{H2N-CH2-COOH <=> H3N^{+}-CH2-COO^{-}} \)
The zwitter ion carries a positive and a negative charge, like the ions of a salt, so glycine shows salt-like character.
Why the other options are incorrect
Option (1) Acetic acid — Incorrect. \( \ce{CH3COOH} \) has no basic group to accept the proton, so no zwitter ion forms; it is only partly ionised in water.
Option (2) Benzoic acid — Incorrect. \( \ce{C6H5COOH} \) is a simple carboxylic acid with no amino group, so it cannot form a dipolar ion.
Option (3) Formic acid — Incorrect. \( \ce{HCOOH} \) also has only an acidic group; it behaves as an ordinary weak acid, not as a salt.
NCERT (2026-27 edition):
10.2.2 Classification of Amino Acids – “Amino acids are usually colourless, crystalline solids. These are water-soluble, high melting solids and behave like salts rather than simple amines or carboxylic acids. This behaviour is due to the presence of both acidic (carboxyl group) and basic (amino group) groups in the same molecule. 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. This is neutral but contains both positive and negative charges.”