By Kevin Beck
Updated Aug 30, 2022
Carbon dioxide (CO₂) is a simple yet profoundly influential molecule. In animals, it is a by‑product of aerobic metabolism; in plants, it is the essential substrate for photosynthesis. Its status as a greenhouse gas has made it a focal point in discussions of climate change, while its versatility keeps it indispensable in countless industrial processes.
Carbon dioxide is a colorless, odorless gas at ambient temperature. Its linear molecular geometry—O=C=O—consists of a single carbon atom double‑bonded to two oxygen atoms. The molecule’s high stability stems from the double bonds that satisfy each atom’s valence requirements.
With a molecular weight of 44 amu (12 for carbon + 2×16 for oxygen), one mole of CO₂—6.02 × 10²³ molecules—has a mass of 44 g. This value follows the convention that 12 g of carbon contains exactly Avogadro’s number of atoms.
CO₂ exists in three states. As a liquid, it is used as a refrigerant and in fire‑extinguishing systems; as a solid (dry ice), it serves in refrigeration and can cause frostbite upon skin contact.
Although CO₂ can be toxic at high concentrations, it is ordinarily a harmless marker of respiration. When breathing ceases, CO₂ accumulates in the bloodstream, leading to asphyxiation—similar to how water is non‑toxic unless it causes drowning.
Atmospherically, CO₂ comprises roughly 0.04% of the air—about 400 ppm today, a sharp rise from the 200–300 ppm range that prevailed for thousands of years before the Industrial Revolution.
In cellular respiration, carbohydrates, proteins, and fats are oxidized to CO₂ and water, producing ATP through the Krebs cycle and electron transport chain. This process powers virtually all aerobic organisms.
CO₂ is a natural greenhouse gas that moderates Earth’s temperature. However, the combustion of fossil fuels—coal, oil, and natural gas—has significantly increased atmospheric CO₂ since the 19th century, accelerating global warming.
Consequences include rising average temperatures, sea‑level rise, glacial melt, ocean acidification, shrinking polar ice, and an uptick in extreme weather events such as hurricanes.
CO₂’s physical properties make it invaluable across industries. It is used as a refrigerant (solid and liquid), an aerosol propellant, a rodenticide, a cryogenic medium in physics experiments, and an enriching gas in greenhouses.
Other applications include hydraulic fracturing of oil wells, mining processes, neutron moderation in certain nuclear reactors, and specialty lasers.
On a personal level, an average adult produces about 500 g of CO₂ daily—roughly one pound of invisible gas—through metabolic activity.
For more detailed information, see NASA’s CO₂ resources and peer‑reviewed chemistry reviews.
