Question

Pure cold conc \(HNO_3\) makes iron passive, as the surface is covered with protective layer of

• A. Fe\(_2\)O\(_3\)
• B. FeO
• C. Fe\(_3\)O\(_4\)
• D. Fe(NO\(_3\))\(_3\)

Hint:

Passivation often forms mixed oxide layer like \(Fe_3O_4\), not just \(Fe_2O_3\).

Answer 1

The Correct Option is C

Concept: Passivation occurs due to formation of a protective oxide layer.

Step 1: Nature of conc \(HNO_3\).
It acts as a strong oxidizing agent.

Step 2: Formation of oxide layer.
\[ \text{Fe} \xrightarrow{\text{conc } HNO_3} \text{Fe}_3\text{O}_4 \]

Step 3: Effect.
This compact oxide layer prevents further reaction → iron becomes passive.