International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

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What is the formula for hydrated iron (III) oxide, the product of rusting?

FeO
Fe2O3·nH2O
Fe3O4
Fe(OH)3

The correct answer is: Fe2O3·nH2O

The formula for hydrated iron (III) oxide, which is commonly associated with rust, is expressed as Fe2O3·nH2O. This notation indicates that iron (III) oxide (Fe2O3) is combined with water molecules, where 'n' represents the number of water molecules attached to the iron oxide in the hydrated form. Rust forms when iron reacts with oxygen in the presence of water or moisture, leading to this hydrated compound, which is crucial in understanding corrosion processes. In contrast, the other options represent different iron compounds that do not accurately depict the rusting process. FeO, or iron (II) oxide, is not involved in rust formation. Fe3O4, known as magnetite, is a mixed oxide of iron and does not represent the hydrated state. Fe(OH)3 is iron (III) hydroxide, which is also a different compound that can occur during the corrosion process but does not specifically indicate the hydrated form of iron (III) oxide, which is the key aspect of rust.