A colorless, odorless, and tasteless gas that is about 1.5 times as heavy as air. Carbon dioxide, composed of one atom of carbon and two atoms of oxygen, makes up about five hundredths of one percent of the air we breathe. Green plants take in carbon dioxide and combine it with water to make sugar. Humans and animals give off carbon dioxide in their exhaled breath.

Even though carbon dioxide makes up only an extremely small part of the air we breathe, it is necessary, because it helps control our rate of breathing. Carbon dioxide is important commercially because of its use as a fire extinguishing agent.