* New Substance Formation: Bronze is a distinct alloy with properties different from either copper or tin. Its composition and structure are unique, resulting in a new material with different hardness, melting point, and color.

* Chemical Bonding: The process of forming bronze involves the formation of new chemical bonds between copper and tin atoms. This rearrangement of atoms at the molecular level signifies a chemical change.

* Irreversibility: While you can melt bronze to separate the copper and tin, you cannot simply reverse the process to go back to the original metals without additional steps.

In contrast, a physical change would be something like:

* Melting a piece of copper (changing state, but still copper).

* Mixing sand and water (no new substances are formed).

Therefore, the formation of bronze is a chemical change because it involves the creation of a new substance with different properties through a change in the chemical composition.