Here's why:

* Substance: A substance is a pure form of matter with a fixed chemical composition. It cannot be broken down into simpler components by physical means.

* Mixture: A mixture is a combination of two or more substances that are not chemically bonded. They can be separated by physical means.

Chlorine (Cl₂) is an element, and its atoms are bonded together to form a diatomic molecule. It has a specific chemical formula and cannot be broken down into simpler components by physical means.