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 $pπ-pπ$ multiple bonds with itself and with other elements having small size and high electronegativity (eg, C, O) Heavier elements of this group do not form $pπ-pπ$ bonds as their atomic orbitals are so large and diffused that they cannot have effective overlapping.

Brown ring in nitrates test is formed when:

$Fe^{2+}$ oxidises

The correct answer is Option (1) → $Fe^{2+}$ oxidises.

The brown ring test is a classic qualitative analysis method used to detect the presence of nitrate ions (\( \text{NO}_3^- \)) in a solution. Here’s a detailed explanation of the process:

Procedure

Preparation: To perform the test, a small amount of the solution suspected of containing nitrates is taken in a test tube.

Addition of Sulfuric Acid: Concentrated sulfuric acid (\( \text{H}_2\text{SO}_4 \)) is carefully added to the test tube. The sulfuric acid is important as it provides the acidic medium necessary for the reaction.

Addition of Ferrous Sulfate: Next, a few drops of ferrous sulfate (\( \text{FeSO}_4 \)) solution are added.

Chemical Reactions

In this acidic medium, \( \text{Fe}^{2+} \) ions can react with the nitrate ions. The reaction involves the oxidation of \( \text{Fe}^{2+} \) ions to \( \text{Fe}^{3+} \) and the reduction of nitrate ions.

The simplified overall reaction can be represented as follows:

\(\text{3Fe}^{2+} + \text{NO}_3^- + \text{4H}^+ \rightarrow \text{3Fe}^{3+} + \text{NO} + \text{2H}_2\text{O}\)

Here, the \( \text{NO}_3^- \) ion is reduced to nitric oxide (\( \text{NO} \)), while the \( \text{Fe}^{2+} \) ion is oxidized to \( \text{Fe}^{3+} \).

Formation of the Brown Ring

The \( \text{Fe}^{3+} \) ions then form a complex with nitric oxide in the presence of water:

\(\text{Fe}^{3+} + \text{NO} + \text{5H}_2\text{O} \rightarrow \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+}\)

This complex is responsible for the characteristic brown ring observed at the interface between the sulfuric acid and the aqueous layer in the test tube.

Summary

Oxidation: The ferrous ions \( \text{Fe}^{2+} \) are oxidized to ferric ions \( \text{Fe}^{3+} \).

Reduction: The nitrate ions \( \text{NO}_3^- \) are reduced to nitric oxide \( \text{NO} \).

Complex Formation: The resulting \( \text{Fe}^{3+} \) ions and \( \text{NO} \) form the brown-colored complex \( \text{[Fe(H}_2\text{O)}_5\text{NO]}^{2+} \).

This test is highly specific for nitrates, making it a reliable method for their detection in various solution