Which gas is colorless and odorless and essential for life?

• A. Oxygen
• B. Carbon Dioxide
• C. Nitrogen
• D. Hydrogen

Answer: (A)

Oxygen powers how cells make energy. In aerobic respiration, cells use oxygen in the mitochondria to convert nutrients into ATP, the energy currency of the cell. That ability to produce energy efficiently is what makes oxygen essential for most life forms. It’s a colorless, odorless gas and makes up about 21% of the air we breathe, so we take it in without noticing it.

Carbon dioxide is also colorless and odorless, but it’s mainly a waste product of metabolism and isn’t used to generate energy for the organism. Nitrogen is abundant in air but is largely inert and not used for energy by most life. Hydrogen plays a crucial role in water and organic molecules, but free hydrogen gas isn’t the energy source living beings rely on in respiration.