Carbon dioxide (chemical formula CO2) is a colorless and odorless gas vital to life on Earth. This naturally occurring chemical compound is composed of a carbon atomcovalently double bonded to two oxygenatoms. Carbon dioxide exists in Earth's atmosphere as a trace gas at a concentration of about 0.04 percent (400 ppm) by volume.[3]Natural sources include volcanoes, hot springs and geysers, and it is freed fromcarbonate rocks by dissolution in water and acids. Because carbon dioxide is soluble in water, it occurs naturally in groundwater,rivers and lakes, in ice caps and glaciers and also in seawater. It is present in deposits ofpetroleum and natural gas.[4]

Atmospheric carbon dioxide is the primary source of carbon in life on Earth and its concentration in Earth's pre-industrial atmosphere since late in the Precambrianwas regulated by photosynthetic organisms and geological phenomena. As part of thecarbon cycle, plants, algae, and cyanobacteriause light energy to photosynthesizecarbohydrate from carbon dioxide and water, with oxygen produced as a waste product.[5]

Carbon dioxide (CO2) is produced by allaerobic organisms when they metabolize carbohydrate and lipids to produce energy byrespiration.[6] It is returned to water via thegills of fish and to the air via the lungs of air-breathing land animals, including humans. Carbon dioxide is produced during the processes of decay of organic materials and the fermentation of sugars in bread, beer andwinemaking. It is produced by combustion ofwood, carbohydrates and fossil fuels such as coal, peat, petroleum and natural gas.

It is a versatile industrial material, used, for example, as an inert gas in welding and fire extinguishers, as a pressurizing gas in air guns and oil recovery, as a chemical feedstock and in liquid form as a solvent in decaffeination of coffee and supercritical drying. It is added to drinking water andcarbonated beverages including beer andsparkling wine to add effervescence. The frozen solid form of CO2, known as "dry ice" is used as a refrigerant and as an abrasive indry-ice blasting.

Carbon dioxide is a significant greenhouse gas. Since the Industrial Revolution, anthropogenic emissions - including the burning of carbon-based fossil fuels and land use changes (primarily deforestation) - have rapidly increased its concentration in the atmosphere, leading to global warming. It is also a major cause of ocean acidificationbecause it dissolves in water to form carbonic acid.[7]

Carbon dioxide has one carbon atom and two oxygen atoms, and a molecular weight of 44 grams per mole ( a certain number of molecules). The oxygen in the airis actually O2, or molecular oxygen, with a molecular weight of 32. Hence, carbon dioxide has a higher density, or is heavier than oxygen.
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