* New Substance: Rust (iron oxide) is a different substance than the original iron in the nail. This is a key indicator of a chemical change.

* Chemical Reaction: The formation of rust is a chemical reaction between iron (Fe), oxygen (O2) from the air, and water (H2O). This reaction forms iron oxide (Fe2O3), the chemical formula for rust.

* Irreversible: While you can remove rust, you can't easily reverse the chemical reaction and turn it back into pure iron.

In contrast, a physical change involves a change in form or appearance but not a change in the chemical composition of the substance. For example, melting ice is a physical change because it's still H2O, just in a different state.