* Homogeneous vs. Heterogeneous: A homogeneous mixture has a uniform composition throughout, meaning the components are evenly distributed. A heterogeneous mixture has a non-uniform composition, with different parts having different properties.

* Chlorine Gas: Chlorine gas is a pure substance, meaning it consists of only one type of molecule (Cl₂). This molecule is evenly distributed throughout the gas, making it uniform in composition.

Therefore, chlorine gas is a homogeneous substance, not a heterogeneous mixture.