Rust on the surface of a metal sheet contains

1. Analyze the question

The question asks for the chemical composition of "rust," which is the reddish-brown flaky substance that forms on the surface of iron or steel when exposed to oxygen and moisture.

2. Explain the concept

Rusting is an electrochemical process that occurs when iron reacts with oxygen in the presence of water. The chemical reaction can be summarized as follows:

• Iron ($Fe$) reacts with Oxygen ($O_2$) and Water ($H_2O$).
• The product formed is a hydrated oxide of iron.
• In this process, iron is oxidized from its metallic state ($Fe^0$) to the iron(III) state ($Fe^{3+}$).

The chemical formula for rust is generally represented as $Fe_2O_3 \cdot xH_2O$. This is known as hydrated iron(III) oxide.

3. Show the solution

• Step 1: Identify the oxidation state of iron in rust. During the corrosion of iron, the iron loses electrons to become $Fe^{3+}$ ions.
• Step 2: Identify the components. The substance is an oxide of iron ($Fe_2O_3$) that has water molecules trapped within its crystal lattice (hydrated).
• Step 3: Evaluate the options: • Option A (Iron(II) oxide) is incorrect because rust involves iron in the +3 oxidation state, not +2. • Option B (Hydrated iron(III) oxide) correctly identifies the +3 oxidation state and the presence of water. • Option C (Iron(III) hydroxide) is a different compound ($Fe(OH)_3$) and is not the primary component of rust. • Option D (Iron(III) trioxocarbonate(IV)) is a carbonate, not the oxide formed during simple rusting.

4. Identify the answer

The correct chemical name for rust is hydrated iron(III) oxide. This matches Option B.

5. Tip

To remember this easily, think of the "3" in Iron(III) as the "rusting" state. Also, remember that rusting requires both oxygen and water; if you keep iron in a dry environment or coat it with oil/paint (to exclude oxygen and water), it will not rust.