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Inorganic compound, a colourless gas with a faint, sharp odour and a sour taste when dissolved in water, chemical formula CO. Constituting about 0.03% of air by volume, it is produced when carbon-containing materials burn completely, and it is a product of fermentation and animal respiration. Plants use CO in photosynthesis to make carbohydrates. CO in Earth's atmosphere keeps some of the Sun's energy from radiating back into space (seegreenhouse effect). In water, CO forms a solution of a weak acid, carbonic acid (HCO). The reaction of CO and ammonia is the first step in synthesizing urea. An important industrial material, CO is recovered from sources including flue gases, limekilns, and the process that prepares hydrogen for synthesis of ammonia. It is used as a refrigerant, a chemical intermediate, and an inert atmosphere; in fire extinguishers, foam rubber and plastics, carbonated beverages (seecarbonation), and aerosol sprays; in water treatment, welding, and cloud seeding; and for promoting plant growth in greenhouses. Under pressure it becomes a liquid, the form most often used in industry. If the liquid is allowed to expand, it cools and partially freezes to the solid form, dry ice.
This entry comes from Encyclopædia Britannica Concise. For the full entry on carbon dioxide, visit Britannica.com.