Yes, there can be impurities in a gas. If we consider the gas, anything that is present except the gas is an impurity. The impurity can be minute solid objects or other gases. If we consider oxygen gas, and add carbon dioxide to it, then carbon dioxide becomes the impurity in oxygen.
A thing is impure if there are two are more types of atoms in it like alt plus sugar etc. actually air is the most impure thing according to me because it contains lot of things like oxygen ,nitrogen, carbon di oxide, nitrogen di oxide, sulphur di oxide, carbon monoxide, neon, argon, krypton, solid particulate matter, vehicles emissions, soil , dust, etc. these are a tuple of impurities in air.
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