What is the chemical formula of rust, which is formed on the surface of iron in the presence of oxygen and moisture?

1. Analyze the question

The question asks for the chemical formula of "rust," the reddish-brown substance that forms on iron when it is exposed to oxygen and water (moisture).

2. Explain the concept

Rusting is a chemical process known as corrosion. It is an electrochemical reaction where iron reacts with oxygen and water to form a hydrated oxide.

The chemical equation for the formation of rust is:

In chemistry examinations, rust is primarily identified as hydrated iron(III) oxide. While the water content ($xH_2O$) varies, the core chemical compound representing the iron oxide component is iron(III) oxide, which has the formula Fe$_2$O$_3$.

3. Show the solution

• Fe$_3$O$_4$ (Option A): This is known as magnetite or iron(II, III) oxide. It is a black magnetic oxide, not the reddish-brown rust we see on iron.
• FeO (Option B): This is iron(II) oxide, also known as wüstite. It is not the primary component of rust.
• Fe(OH)$_3$ (Option C): This is iron(III) hydroxide. While it can be a product in the intermediate stages of corrosion, it is not the standard chemical formula for rust.
• Fe$_2$O$_3$ (Option D): This is iron(III) oxide. This is the correct chemical representation of the oxide layer formed during the rusting of iron.

4. Identify the answer

Comparing our analysis with the options provided, Fe$_2$O$_3$ is the correct chemical formula for rust.

5. Tip

To remember this easily, think of the "III" in iron(III) oxide. Iron has a valency of 3, and Oxygen has a valency of 2. By swapping the valencies (criss-cross method), you get Fe$_2$O$_3$.