Corrosion chemistry: When an iron nail rusts, which oxide compound is actually formed? Identify the specific iron oxide produced during rusting (under atmospheric moisture and oxygen). Choose the correct chemical name/formula.

Given data

• Metal: Iron (nail) exposed to air and moisture.
• Phenomenon: Rusting (electrochemical corrosion).

Concept/Approach
Rust is not a single, simple oxide like FeO or Fe₂O₃. It is predominantly hydrated ferric oxide, commonly represented as Fe₂O₃·nH₂O.

Reasoning
• Moisture provides electrolyte → oxidation of Fe to Fe²⁺/Fe³⁺. • Oxygen reduces to hydroxide ions; hydrated ferric oxide forms and deposits as rust (reddish-brown).

Common pitfalls
Confusing rust with magnetite (Fe₃O₄) or anhydrous Fe₂O₃; these may occur in scales but typical atmospheric rust is hydrated ferric oxide.

Final Answer
Hydrated ferric oxide (Fe₂O₃·nH₂O)