The reddish–brown rust on iron roofing sheets consists of

Analysis of the Question

This question tests your knowledge of corrosion, specifically the chemical composition of rust. You are asked to identify the chemical formula for the reddish-brown substance that forms on iron when it is exposed to oxygen and moisture.

Concept Explanation

Rusting is an electrochemical process that occurs when iron reacts with oxygen in the presence of water. The chemical name for rust is hydrated iron(III) oxide.

• Iron(III) oxide ($Fe_2O_3$): This is the base compound formed when iron oxidizes.
• Hydrated ($XH_2O$): The "X" represents a variable number of water molecules that are chemically attached to the iron oxide structure. This is why the formula is written with $XH_2O$ rather than a fixed number.

Step-by-Step Solution

1. Identify the components: Rust is formed from Iron ($Fe$), Oxygen ($O_2$), and Water ($H_2O$).
2. Determine the oxidation state: In rust, iron exists in the +3 oxidation state, forming $Fe_2O_3$.
3. Account for hydration: Because rust is always hydrated (contains water), we add the $XH_2O$ suffix.
4. Evaluate the options: • Option A represents a complex ion, not rust. • Option B is an incorrect representation of iron oxide. • Option C ($Fe_2O_3.XH_2O$) correctly represents hydrated iron(III) oxide. • Option D is chemically incorrect and contains a typo in the formula.

Exam Strategy Tip

When you see questions about "rusting" in Chemistry, always look for the combination of Iron(III) oxide ($Fe_2O_3$) and water ($H_2O$). Remember that the "X" is used because the amount of water trapped in the rust can vary depending on the humidity of the environment.