* New substance formed: Rust (iron oxide) is a completely different substance than the original iron in the nail.

* Irreversible: You can't easily reverse the process and turn rust back into pure iron.

* Chemical reaction: Rusting happens when iron reacts with oxygen and water, forming a new compound with different properties.

Physical changes involve changes in form or appearance but not the composition of the substance. Examples include melting ice, cutting paper, or crushing a rock.