Step 1: Justification for Zinc Oxide as an Amphoteric Oxide:
Zinc oxide (ZnO) exhibits amphoteric behavior, reacting with both acids and bases. It forms salts and water upon reaction with acids, and zincates when reacting with bases.
1. Reaction with an acid (e.g., hydrochloric acid) yields zinc chloride and water:
\[
\text{ZnO} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O}
\]
2. Reaction with a base (e.g., sodium hydroxide) produces sodium zincate and water:
\[
\text{ZnO} + 2\text{NaOH} + \text{H}_2\text{O} \rightarrow \text{Na}_2\text{ZnO}_2 + 2\text{H}_2\text{O}
\]
This dual reactivity with acids and bases categorizes zinc oxide as amphoteric.
Step 2: Rationale for Storing Sodium Metal in Kerosene Oil:
Sodium metal's high reactivity, particularly with water and atmospheric oxygen, necessitates special storage. Its vigorous reaction with moisture produces sodium hydroxide and hydrogen gas, posing a safety hazard.
Consequently, sodium metal is submerged in kerosene oil to isolate it from air and moisture. The kerosene acts as a protective shield, inhibiting reactions with water and oxygen.
Step 3: Explanation for Absence of Hydrogen Gas Evolution in Nitric Acid-Metal Reactions:
Nitric acid (HNO₃) typically does not evolve hydrogen gas when reacting with metals due to its potent oxidizing properties. Instead of releasing hydrogen, nitric acid oxidizes the metal to form metal nitrates, with nitrogen oxides such as nitrogen dioxide (NO₂), nitric oxide (NO), or nitrous oxide (N₂O) being produced. For instance, zinc reacts with nitric acid to form zinc nitrate and nitrogen dioxide gas:
\[
\text{Zn} + 2\text{HNO}_3 \rightarrow \text{Zn(NO}_3)_2 + \text{NO}_2 + \text{H}_2\text{O}
\]
Thus, the oxidizing nature of nitric acid prevents the release of hydrogen gas.
Step 4: Summary:
- Zinc oxide is classified as amphoteric due to its reactions with both acids and bases.
- Sodium metal is stored in kerosene oil to prevent contact with moisture and air.
- Hydrogen gas is not evolved in nitric acid-metal reactions because nitric acid acts as an oxidizing agent, generating nitrogen oxides instead.