Here's why:

* Chemical Change: Tarnishing involves the metal reacting with substances in the environment, forming a new compound on the surface. This is a chemical change, as the original metal is transformed into something different.

* Oxidation: Tarnishing is often caused by oxidation, where the metal reacts with oxygen in the air. For example, copper tarnishes to form copper oxide, which has a greenish color.

* Corrosion: While not all tarnishing is corrosion, it is a form of surface degradation. Corrosion is a more general term that refers to the deterioration of a material due to chemical reactions with its environment.

Let me know if you'd like more details about specific types of tarnishing!