Which of the following can be classified as a mixture?

a

pure air

b

pure water

c

pure nitrogen

d

pure gold

A pure substance is either an element (e. g. carbon, oxygen, nitrogen, sodium, lead etc.) or a compound (e. g. H2O, CO2, NH3, NaCl etc.). Air comprises elements such as N2, O2, argon and also compounds such as CO2, H2O (water vapour) etc ... Hence, air is amixture and not a pure substance.

A. Pure air

It's classified as a mixture since it's a uniform physical combination of gases such as nitrogen, oxygen, carbon dioxide, and argon