* New Substance Formation: When copper reacts with oxygen and water in the air, it forms a new substance called copper carbonate (also known as patina). This green substance has different chemical properties than the original copper.

* Irreversible Process: The formation of copper carbonate is a relatively permanent change. You can't easily turn the green patina back into pure copper without a chemical reaction.

* Chemical Reaction: The process involves a chemical reaction between copper, oxygen, and water. This reaction forms new chemical bonds and alters the composition of the material.

In summary: The greening of copper is a chemical change because it results in the formation of a new substance with different properties through a chemical reaction.