carbon dioxide

Inorganic compound, a colourless gas with a faint, sharp odour and a sour taste when dissolved in water, chemical formula CO₂. Constituting about 0.03percnt 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 CO₂ in photosynthesis to make carbohydrates. CO₂ in Earth's atmosphere keeps some of the Sun's energy from radiating back into space (see greenhouse effect). In water, CO₂ forms a solution of a weak acid, carbonic acid (H₂CO₃). The reaction of CO₂ and ammonia is the first step in synthesizing urea. An important industrial material, CO₂ 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 (see carbonation), 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.

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