Question

Question #eb6b2

Answer 1

Carbon Dioxide (`CO_2`) is a gas consisting of a single kind of molecule, which makes it a pure substance. A Carbon dioxide molecule is made from one atom of Carbon and two atoms of Oxygen and this compound under normal conditions exists as a gas.

Normal air or "atmosphereric air" is a mixture of gases, including Carbon Dioxide, that is retained on earth by the planet's gravity and it's essential for life. It contains 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide, and small amounts of other gases including helium,argon etc.
Hence it is observed that the air around us also contains Carbon Dioxide and that Carbon Dioxide gas is a subset of the larger gaseous mixture we call Air.

Source & further reading:
Atmosphere of Earth

Answer 2

Carbon dioxide is a pure substance consisting only of carbon dioxide molecules.

Air, as indicated in a previous answer, is not a pure substance, but is a mixture of gases consisting of 78% nitrogen gas molecules, 21% oxygen gas molecules, 0.93% argon gas atoms, 0.038% carbon dioxide molecules, with the remainder molecules of water vapor and trace gases.
http://m.space.com/17683-earth-atmosphere.html