Chlorine gas is not a homogeneous mixture. It is a pure substance, specifically an element.

Here's why:

* Homogeneous mixture: A homogeneous mixture is a combination of two or more substances where the composition is uniform throughout. Examples include saltwater or air.

* Pure substance: A pure substance is a single type of matter with a fixed composition. Elements and compounds are considered pure substances.

* Element: An element is a pure substance made up of only one type of atom. Chlorine (Cl) is an element.

Therefore, chlorine gas (Cl₂) is composed solely of chlorine atoms and is considered a pure substance, not a mixture.