* New substance formation: Rust (iron oxide) is a completely different substance than the original iron in the nail. It has a different chemical composition (Fe2O3) and different properties (color, texture, etc.).
* Irreversible process: You can't easily turn rust back into iron. To do so, you'd need a chemical reaction (like smelting).
* Chemical bonds are broken and formed: The iron in the nail reacts with oxygen and water in the air, breaking the bonds in the original iron and forming new bonds with oxygen to create rust.
In contrast to a physical change:
* A physical change involves a change in form or appearance but not a change in chemical composition. For example, melting ice is a physical change (water changes state from solid to liquid). The chemical makeup of the water (H2O) remains the same.
Therefore, rusting is a chemical change because it involves the formation of a new substance with a different chemical composition.
