Answer the question on basis of passage given below:
Nitrogen differs from the rest of the members of its group due to its small size, high electronegativity, high ionisation enthalpy and non-availability of d-orbitals. Nitrogen has unique ability to form \(p\pi-p\pi\) multiple bonds with itself and with other elements having small size and high electronegativity (e.g., C, O). Heavier elements of this group do not form \(p\pi-p\pi\) bonds as their atomic orbitals are so large and diffused that they cannot have effective overlapping.
Brown ring in nitrates test is formed when:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(Fe^{2+}\) is oxidised
In the brown ring test, freshly prepared \( \ce{FeSO4} \) solution is added to the nitrate solution and concentrated \( \ce{H2SO4} \) is poured carefully down the side of the test tube. Fe2+ reduces the nitrate to nitric oxide and is itself oxidised to \( \ce{Fe^{3+}} \); the NO formed then binds to more \( \ce{Fe^{2+}} \) and gives the brown complex at the junction of the two layers:
\( \ce{NO3^{-}} \) + \( \ce{3Fe^{2+}} \) + \( \ce{4H^{+}} \) → NO + \( \ce{3Fe^{3+}} \) + \( \ce{2H2O} \)
\( \ce{[Fe(H2O)6]^{2+}} \) + NO → \( \ce{[Fe(H2O)5(NO)]^{2+}} \) (brown) + \( \ce{H2O} \)
Iron goes from +2 to +3, so the ring is formed when Fe2+ is oxidised, while nitrogen goes from +5 (\( \ce{NO3^{-}} \)) to +2 (NO).
Why the other options are incorrect
Option (2) \(Fe^{3+}\) is reduced — Incorrect. \( \ce{Fe^{3+}} \) is a product of the test, not the reagent; the iron changes from +2 to +3, which is oxidation, not reduction.
Option (3) \(NO_3^-\) is oxidised — Incorrect. Nitrogen falls from +5 in \( \ce{NO3^{-}} \) to +2 in NO, so the nitrate is reduced; it is the oxidising agent of the test.
Option (4) \(HNO_3\) is oxidised — Incorrect. Nitrogen in \( \ce{HNO3} \) is already in its highest oxidation state (+5), so it cannot be oxidised further; the nitrate is reduced to NO.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Brown Ring Test – “The familiar brown ring test for nitrates depends on the ability of Fe2+ to reduce nitrates to nitric oxide, which reacts with Fe2+ to form a brown coloured complex. The test is usually carried out by adding dilute ferrous sulphate solution to an aqueous solution containing nitrate ion, and then carefully adding concentrated sulphuric acid along the sides of the test tube. A brown ring at the interface between the solution and sulphuric acid layers indicate the presence of nitrate ion in solution.”