Carbon Dioxide, colorless, odorless, and slightly acid-tasting gas, sometimes called carbonic acid gas, the molecule of which consists of one atom of carbon joined to two atoms of oxygen (CO2). It was called "fixed air" by the Scottish chemist Joseph Black, who obtained it through the decomposition of chalk and limestone and recognized that it entered into the chemical composition of these substances. The French chemist Antoine Lavoisier proved that it is an oxide of carbon by showing that the gas obtained by the combustion of charcoal is identical in its properties with the "fixed air" obtained by Black. Carbon dioxide is about 1.5 times as dense as air. It is soluble in water, 0.9 volume of the gas dissolving in 1 volume of water at 20° C (68° F).

Carbon dioxide is produced in a variety of ways: by combustion, or oxidation, of materials containing carbon, such as coal, wood, oil, or foods; by fermentation of sugars; and by decomposition of carbonates under the influence of heat or acids. Commercially, carbon dioxide is recovered from furnace or kiln gases; from fermentation processes; from reaction of carbonates with acids; and from reaction of steam with natural gas, a step in the commercial production of ammonia. The carbon dioxide is purified by dissolving it in a concentrated solution of alkali carbonate or ethanolamine and then heating the solution with steam. The gas is evolved and is compressed into steel cylinders.

The atmosphere contains carbon dioxide in variable amounts, usually 3 to 4 parts per 10,000, and has been increasing by 0.4 percent a year. It is used by green plants in the process known as photosynthesis, by which carbohydrates are manufactured .

Carbon dioxide is used in the manufacture of sodium carbonate, Na2CO3 · 1OH2O (washing soda); sodium bicarbonate, NaHCO3 (baking soda); and basic carbonate of lead, Pb3 (OH)2(CO3)2 (white lead). Dissolved under a pressure of 2 to 5 atmospheres, carbon dioxide causes the effervescence in carbonated beverages. Carbon dioxide does not burn and does not support ordinary combustion, and because of these properties it is used for extinguishing fires. The CO2 extinguisher is a steel cylinder filled with liquid carbon dioxide, which, when released, expands suddenly and causes so great a lowering of temperature that it solidifies into powdery "snow." This snow volatilizes (vaporizes) on contact with the burning substance, producing a blanket of gas that cools and smothers the flame. Solid carbon dioxide, known as dry ice, is widely used as a refrigerant. Its cooling effect is almost twice that of water ice; its special advantages are that it does not melt as a liquid but turns into gas, and that it produces an inert atmosphere that reduces bacterial growth.

The presence of carbon dioxide in the blood stimulates breathing. For this reason, carbon dioxide is added to oxygen or ordinary air in artificial respiration and to the gases used in anesthesia.

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