Question

Why can air be treated as a pure substance if it is composed of multiple gases?

Answer 1

Air is a homogeneous mixture, so it can act like a pure substance.

Answer 2

As long it it stays a gas, (above boiling point of N2 and O2), the nature of the molecules is indifferent to the physical behaviour.

This would be true for any gas mixture.

Answer 3

Air in the atmosphere is a mixture of gases where 78 percent is nitrogen , oxygen 21 percent and other gases are present such as carbon dioxide,hydrogen,methane, and other inert gases and this gases are in proportion that they appear as pure substance.