Air is considered a mixture because of the following reasons:

(i) Air can be separated into its constituents like oxygen, nitrogen, etc., by the physical process of fractional distillation.

(ii) Air shows the properties of all the gases present in it. For example, oxygen supports combustion and air also supports combustion; carbondioide turns lime water milky and air also turns lime water milky, though very, very slowly.

(iii) Air has a variable composition because air at different places contains different amounts of the various gases.