Here's why:

* Substance: A pure form of matter with a definite and constant composition. It cannot be broken down into simpler substances by physical means.

* Mixture: A combination of two or more substances where each substance retains its own properties.

Oxygen (O₂) is a chemical element and exists as a diatomic molecule. It has a fixed composition and cannot be broken down into simpler substances by physical means like filtering or evaporation.