Rust is the common name for which compound?

• A. Iron oxide without water
• B. Iron carbonate
• C. Iron hydroxide
• D. Hydrated iron oxide

Answer: (D)

Rust reflects the hydrated iron oxide that forms when iron corrodes in the presence of water and oxygen. The water isn’t just a solvent in this process; it becomes part of the solid product, giving a formula often represented as Fe2O3·nH2O. That water content is why the material is called a hydrated oxide and why rust has its characteristic flaky, porous texture and reddish-brown color. Because of this, rust is best described as hydrated iron oxide rather than a pure iron oxide, an iron carbonate, or iron hydroxide. Iron carbonate would require carbonate ions to react with iron, which isn’t the typical rusting pathway, and iron hydroxide is more of a transient intermediate or a separate compound, not the common rust product.