Here's why:

* Pure substances are made up of only one type of molecule. Examples include water (H₂O) and oxygen (O₂).

* Mixtures are combinations of two or more substances that are not chemically bonded.

Tap water contains:

* Water (H₂O): This is the main component.

* Dissolved minerals: These include calcium, magnesium, sodium, and chloride.

* Other substances: Tap water can also contain traces of chlorine, fluoride, and other chemicals added for treatment.

Because tap water contains multiple substances, it is classified as a mixture.