carbon cycle Circulation through nature of carbon in the form of the simple element and its compounds. The source of carbon in living things is carbon dioxide (C0 2 ) from air or dissolved in water. Algae and green plants (producers) use C0 2 in photosynthesis to make carbohydrates, which in turn are used in the processes of metabolism to make all other compounds in their tissues and those of animals that consume them. The carbon may pass through several levels of herbivores and carnivores (consumers). Ani¬ mals and, at night, plants return the C0 2 to the atmosphere as a by-product of respiration. The carbon in animal wastes and in the bodies of organisms is released as C0 2 in a series of steps by decay organisms (decomposers), chiefly bacteria and fungi (see fungus). Some organic carbon (the remains of organisms) has accumulated in Earth’s crust in fossil fuels, limestone, and coral. The carbon of fossil fuels, removed from the cycle in prehis¬ toric times, is being returned in vast quantities as C0 2 via industrial and agricultural processes, some accumulating in the oceans as dissolved car¬ bonates and some staying in the atmosphere (see greenhouse effect).
carbon dioxide Inorganic compound, a colourless gas with a faint, sharp odour and a sour taste when dissolved in water, chemical formula C0 2 . Constituting about 0.03% of air by volume, it is produced when carbon-containing materials burn completely, and it is a product of fer¬ mentation and animal respiration. Plants use C0 2 in photosynthesis to make carbohydrates. C0 2 in Earth’s atmosphere keeps some of the Sun’s energy from radiating back into space (see greenhouse effect). In water, C0 2 forms a solution of a weak acid, carbonic acid (H 2 C0 3 ). The reaction of C0 2 and ammonia is the first step in synthesizing urea. An important industrial material, C0 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.
carbon-14 dating or radiocarbon dating Method of determin¬ ing the age of once-living material, developed by U.S. physicist Willard Libby in 1947. It depends on the decay of the radioactive isotope carbon-14 (radiocarbon) to nitrogen. All living plants and animals continually take in carbon: green plants absorb it in the form of carbon dioxide from the atmosphere, and it is passed to animals through the food chain. Some of this carbon is radioactive carbon-14, which slowly decays to the stable isotope nitrogen-14. When an organism dies it stops taking in carbon, so the amount of carbon-14 in its tissues steadily decreases. Because carbon-14 decays at a constant rate, the time since an organism died can be estimated by measuring the amount of radiocarbon in its remains. The method is a useful technique for dating fossils and archaeological speci¬ mens from 500 to 50,000 years old and is widely used by geologists, anthropologists, and archaeologists.
carbon monoxide Inorganic compound, a highly toxic, colourless, odourless, flammable gas, chemical formula CO. It is produced when car¬ bon (including coal and coke) or carbon-containing fuel (including petro¬ leum hydrocarbons; e.g., gasoline, fuel oil) does not burn completely to carbon dioxide, because of insufficient oxygen. CO is present in the exhaust gases of internal combustion engines and furnaces. It is toxic because it binds to hemoglobin in blood much more strongly than does oxygen and thus interferes with transport of oxygen from lungs to tissues (see hypoxia; respiration). Symptoms of CO poisoning range from headache, nausea, and syncope to coma, weak pulse, respiratory failure, and death. CO is used industrially as a fuel and in synthesis of numerous organic compounds, including methanol, ethylene, and aldehydes.
carbon steel Alloy of iron and carbon in which the carbon content may range from less than 0.015% to slightly more than 2%. Adding this tiny amount of carbon produces a material that exhibits great strength, hard¬ ness, and other valuable mechanical properties. Carbon steels account for about 90% of the world’s steel production. They are used extensively for automobile bodies, appliances, machinery, ships, containers, and the structures of buildings. Carbon steel, formerly made by the Bessemer, cru¬ cible, or open-hearth process, is now made by the basic oxygen process, or by an arc furnace.
Carbonari \,kar-b3-'na-re\ (Italian dialect: “Charcoal Burners”) Mem¬ bers of a secret society (the Carbonaria) in early-19th-century Italy. Advo¬ cating liberal and patriotic ideas, the Carbonari favored constitutional and representative government and aimed to protect Italian interests against foreigners. They helped lead the unsuccessful revolts of 1820 and 1831 and were gradually absorbed into the Young Italy movement. Their influ¬ ence prepared the way for the Risorgimento.
carbonate Any member of two classes of chemical compounds—one inorganic and the other organic—that are derived from carbon dioxide (C0 2 ) or its water solution, carbonic acid (H 2 C0 3 ). Inorganic carbonates (MC0 3 or M 2 C0 3 , where M is a metal atom of, e.g., calcium or sodium) are salts of carbonic acid. The shells and other hard parts of shellfish are calcium carbonate, as is the limestone they turn into. Many other miner¬ als, including calcite, dolomite, and aragonite, consist of or contain car¬ bonates. Sodium carbonate is one of the world’s most important basic chemical commodities. Organic carbonates are esters of carbonic acid and various alcohol groups (methyl, ethyl, or phenyl). These are liquids used as solvents and to synthesize plastics and other compounds.
carbonate mineral Any member of a family of minerals that con¬ tains the carbonate ion, C0 3 2- , as the basic structural unit. The carbon¬ ates are among the most widely distributed minerals in the earth’s crust; the most common are calcite, dolomite, and aragonite. Dolomite replaces calcite in limestones; when this replacement is extensive, the rock is called dolomite. Other relatively common carbonate minerals are siderite, rhodo- chrosite, strontianite (strontium-rich); smithsonite (zinc-rich); witherite (barium-rich); and cerussite (lead-rich).
carbonation Addition of carbon dioxide gas to a beverage, imparting sparkle and a tangy taste and preventing spoilage. The liquid is chilled and cascaded down in an enclosure containing carbon dioxide (either as dry ice or a liquid) under pressure. Increasing pressure and lowering tem¬ perature maximize gas absorption. Carbonated beverages do not require PASTEURIZATION.