Rust's formula can be represented as Fe2O3·xH2O. Which of the following resembles rust's formula?

• A. FeO
• B. Fe2O3
• C. Fe3O4
• D. Fe2O3·xH2O

Answer: (D)

Rust isn’t just a simple iron oxide; it forms as iron oxide that has water incorporated into its solid structure. That hydration is captured by Fe2O3·xH2O, where x denotes how many water molecules are associated per formula unit. This water-of-hydration is a distinguishing feature of rust, reflecting that the corrosion product forms in the presence of moisture. The other forms represent anhydrous or different iron oxides: Fe2O3 alone is iron(III) oxide without water; FeO is iron(II) oxide; Fe3O4 is magnetite, an oxide with a different iron oxidation state mix. So the representation Fe2O3·xH2O best matches rust because it explicitly includes water of hydration.