Here's why:
* Solutions are homogeneous mixtures where one substance (solute) dissolves completely into another (solvent). For example, salt dissolved in water. Chlorine doesn't dissolve in this way.
* Colloids are mixtures where tiny particles of one substance are suspended in another. Think of milk, where fat droplets are suspended in water. Chlorine doesn't form these kinds of suspensions.
Chlorine exists as a diatomic molecule (Cl₂) in its gaseous form and as a liquid under pressure. It can react with other substances to form compounds (like bleach, which is a solution), but chlorine itself is an element.
