* New substance formed: Rust (iron oxide) is a completely different substance from iron. It has a different chemical composition, color, and properties. This indicates a chemical reaction has taken place.
* Irreversible (without further chemical reactions): You can't simply reverse the process and turn rust back into iron by physical means like heating or cooling. You'd need a chemical reaction, like reduction.
* Evidence of a reaction: Rusting involves the reaction of iron with oxygen and water. This is a chemical reaction, and the reaction releases heat (although not noticeably).
* Change in properties: Rust is brittle and crumbly, unlike the strong, metallic iron nail. This change in properties is another indicator of a chemical change.
In summary: Rusting is a chemical change because it involves the formation of a new substance (iron oxide) with different properties than the original substance (iron). This change is irreversible without further chemical reactions.
