* Rust Formation: Rust is iron oxide, a new substance formed when iron (in the bike) reacts with oxygen (in the air) and water (from the rain).

* Irreversible Change: The process of rusting is a chemical reaction that changes the composition of the iron. The original iron atoms are bonded to oxygen atoms, creating a new compound (iron oxide). This change is irreversible.

* New Substance: Rust has different properties than iron. It's brittle, reddish-brown, and doesn't conduct electricity as well. This indicates a new substance has been formed.

In contrast, a physical change doesn't alter the composition of a substance. For example, melting ice is a physical change because it's still water (H₂O) in a different state.