Rusting of Iron: Requirements and Prevention

Steps:

• Rusting of iron is an electrochemical process requiring both oxygen (O₂) and water (H₂O) for oxidation and reduction reactions.
• Sacrificial protection uses a more reactive metal than iron, which corrodes instead, preventing iron rusting.
• Evaluate options: Check if conditions match O₂ + H₂O and if the metal is more reactive (e.g., magnesium in reactivity series above iron).
• Select option with both correct conditions and suitable metal.

Why D is correct:

• Rusting follows 4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃, needing O₂ + H₂O; magnesium (Mg) is above Fe in reactivity series, so Mg oxidizes preferentially (Mg → Mg²⁺ + 2e⁻).

Why the others are wrong:

• A: O₂ alone insufficient; copper below Fe, cannot protect sacrificially.
• B: O₂ alone insufficient; magnesium correct for protection but conditions wrong.
• C: H₂O alone insufficient; copper below Fe, cannot protect sacrificially.

Final answer: D