Inorganic compound, a colourless gas with a faint, sharp odour and a sour taste when dissolved in water, chemical formula CO 2 .
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 2 in photosynthesis to make carbohydrate s. CO 2 in Earth's atmosphere keeps some of the Sun's energy from radiating back into space (see greenhouse effect ). In water, CO 2 forms a solution of a weak acid , carbonic acid (H 2 CO 3 ). The reaction of CO 2 and ammonia is the first step in synthesizing urea . An important industrial material, CO 2 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 .