Here's why:

* Pure substances have a fixed chemical composition and a definite set of properties.

* Mixtures are combinations of two or more substances that are not chemically bonded and can be separated by physical means.

KCl is a chemical compound formed by the ionic bond between potassium (K) and chlorine (Cl). It has a consistent chemical formula (KCl) and a specific set of properties like melting point, boiling point, and solubility.

Therefore, KCl is a pure substance, not a mixture.