Air is a mixture because:
(i) The properties of the constituents of air (oxygen, nitrogen, carbon dioxide etc.) are not changed. For example, pure oxygen is a supporter of combustion. Oxygen present in air also supports combustion. Pure carbon dioxide turns lime water milky. Carbon dioxide present in air also turns lime water milky.
(ii) The proportions of the constituents of air vary from place to place. Even in the same place the proportion varies from time to time. In a compound, the proportions of different constituents always remain fixed.
(iii) On distillation of liquefied air, nitrogen distills off first, leaving behind oxygen in the liquid form. This is the characteristic of a mixture, not of a compound.
(iv) No thermal change occurs when the constituents of air (oxygen, nitrogen etc.) are mixed together.
(v) Liquid air does not have a fixed boiling point. This is the characteristic of a mixture, not of a compound.