Which of the following statements are correct?
A. All amino acids contain amino \(-NH_2\) and \(-COOH\) functional groups.
B. All amino acids are optically active.
C. All amino acids are solid at room temperature.
D. In aqueous solution amino acids exist as Zwitter ions.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, C, D only
A. All amino acids contain amino (–NH2) and –COOH functional groups — Correct. Amino acids are, by definition, compounds containing an amino group and a carboxyl group.
B. All amino acids are optically active — Incorrect. Glycine (R = H) has two H atoms on the α-carbon, so its α-carbon is not asymmetric and it is optically inactive. All other naturally occurring α-amino acids are optically active.
C. All amino acids are solid at room temperature — Correct. Amino acids are colourless, crystalline, high-melting solids that behave like salts.
D. In aqueous solution amino acids exist as zwitter ions — Correct. The –COOH group loses a proton and the –\( \ce{NH2} \) group accepts one, giving the dipolar ion \( \ce{H3N^{+}-CHR-COO^{-}} \).
NCERT (2026-27 edition):
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. In zwitter ionic form, amino acids show amphoteric behaviour as they react both with acids and bases. Except glycine, all other naturally occurring α-amino acids are optically active, since the α-carbon atom is asymmetric.”