Definition of Air in Science

The term "air" refers generally to gas, but exactly which gas depends on the context in which the term is used. Let's learn about the modern definition of air in scientific disciplines and the earlier definition of the term.

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How to Define Atmosphere

By Anne Marie Helmenstine, Ph.D.

Modern Air Definition

Air is the general name for the mixture of gases that makes up the Earth's atmosphere. This gas is primarily nitrogen (78%), mixed with oxygen (21%), water vapor (variable), argon (0.9%), carbon dioxide (0.04%), and trace gases. Pure air has no discernible scent and no color. Air typically contains dust, pollen, and spores; other contaminants are referred to as "air pollution." On another planet—Mars, for example—the so-called air would have a different composition since there is technically no air in space.

Older Air Definition

Air is also an early chemical term for a type of gas. In the older definition, many individual types of so-called air made up the air we breathe: Vital air was later determined to be oxygen; what was called phlogisticated air turned out to be nitrogen. An alchemist might refer to any gas released by a chemical reaction as its "air."