Air

Air, heterogeneous mixture of tasteless, odorless, colorless, and invisible gases surrounding the earth, consisting of about 78% nitrogen, 21% oxygen, and 1% argon, carbon dioxide, hydrogen, krypton, neon, helium, and xenon. Air is what we breathe and what is essential to all plant and animal life. It is kept close to the surface of the planet by the force of gravity.

See also: Air pollution; Climate; Gas; Nitrogen; Oxygen; Weather.