Carbon dioxide is a colorless, odorless gas. It occurs in the atmospheres of many planets, including that of the earth. On the earth, all green plants must get carbon dioxide from the atmosphere to live and grow. Animals produce the gas when their bodies convert food into energy and living tissue. Animals release carbon dioxide into the atmosphere. Carbon dioxide is also created by the burning of any substance that contains carbon. Such substances include coal, gasoline, and wood. Fermentation and the decay of plants and animals also produce carbon dioxide (see Fermentation ). Carbon dioxide makes up less than 1 percent of the earth’s atmosphere.
The carbon dioxide in the earth’s atmosphere helps regulate the planet’s temperature. When sunlight reaches the earth, some of it is converted into heat. Carbon dioxide absorbs some of the heat and so helps keep it near the earth’s surface. If all the heat from sunlight escaped into outer space, the earth would become very cold. The amount of carbon dioxide in the atmosphere has been increasing since about the early to middle 1800’s, chiefly as a result of the burning of fuels that contain carbon. This increase has caused a slight rise in the earth’s average temperature. See Greenhouse effect .
Carbon dioxide has important uses in the home and in industry. For example, carbon dioxide released by baking powder or yeast makes cake batter rise. Carbon dioxide in soft drinks, beer, and sparkling wines gives the beverages their fizz. Some fire extinguishers use carbon dioxide because it does not burn and because pure carbon dioxide is heavier than air. Carbon dioxide’s heaviness enables it to blanket a fire and prevent oxygen in the air from reaching the fire. Fires need oxygen to continue burning. Carbon dioxide’s heaviness can also cause it to collect at the bottom of caves, mines, silos, and wells. Carbon dioxide may keep oxygen out of such places and so make it dangerous for people and animals to breathe there.
The gas becomes a solid at -109.3 °F (-78.5 °C). Solid carbon dioxide is commonly called dry ice because, at normal pressures, it does not become a liquid as its temperature rises. Instead, it sublimes–that is, it changes from a solid directly into a gas.
Carbon dioxide molecules consist of one carbon atom and two oxygen atoms. The gas was first identified in the 1750’s by Joseph Black, a Scottish chemist and physician.
