Here's why:

* Pure substances are made up of only one type of atom or molecule. Examples include oxygen (O2) or nitrogen (N2).

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

Clean air contains a variety of gases, including:

* Nitrogen (N2) - the most abundant gas

* Oxygen (O2)

* Argon (Ar)

* Carbon dioxide (CO2)

* Trace amounts of other gases

Since clean air contains multiple different substances, it is classified as a mixture.