A chemical element that, at normal atmospheric temperatures and pressures, exists as a colorless, odorless and tasteless gas and comprises about 21% of the earth's atmosphere by volume is described as which gas?

• A. Helium
• B. Oxygen
• C. Nitrogen
• D. Nitrous oxide

Answer: (D)

The statement describes the gas that makes up about 21% of Earth's atmosphere at room conditions, and that gas is oxygen. Air is roughly 78% nitrogen and about 21% oxygen, with trace amounts of other gases. Oxygen is colorless, odorless, and tasteless, and it is essential for most aerobic life. Nitrogen, while also colorless and odorless, is the major component of air, not the ~21% portion. Nitrous oxide is present only in trace amounts in the atmosphere and is used medically as an anesthetic; it does not account for the 21% of the air.