Carbon dioxide, a greenhouse gas, is by far the most common compound of carbon. It comprises just a small fraction of the Earth's atmosphere, yet it is vital to life because of its role in photosynthesis and in the formation of all biological structure. Carbon dioxide can be produced by combustion of carbon and carbon-containing compounds in excess oxygen by this chemical reaction: CaCO3 (calcium carbonate) + O2 (oxygen) = CaO (calcium oxide) + CO2. Another method is reaction of a weak acid solution with a metal carbonate: CaCO3 + 2H+ (hydrogen ion) = Ca2+ (Calcium ion) + CO2 + H2O.
Carbon monoxide is a highly toxic gas; nevertheless, it has important uses and is produced on an industrial scale principally by passing steam over red-hot coke to yield "water gas," a mixture of carbon monoxide and hydrogen gases. The reaction is: C + H2O = CO + H2.
Another important method is the high-temperature reaction of methane with steam to yield "synthesis gas": CH4 + H2O = CO + 3H2. The heating of elemental carbon in a limited supply of oxygen, a process called pyrolysis, leads to the chemical reaction: C + O2 = 2CO.
Carbon disulfide is a toxic, flammable liquid. This compound is used principally as a solvent for sulfur and rubber, and, at a declining rate, in the production of fluorocarbon propellants and refrigerants. When heated, carbon and sulfur combine to yield carbon disulfide by the chemical reaction: C + 2S = CS2.
Carbon tetrachloride, a toxic liquid, is widely used as a solvent for oil and grease and as a cleaning agent. It is produced when carbon disulfide (CS2) gas is heated with chlorine (CL) gas, according to the reaction: CS2 + 3CL2 = CCL4 + S2CL2 (disulfur dichloride gas).
Some other common compounds of carbon are carbon tetrachloride (CCl4), methane (CH4), ethylene (C2H4), acetylene (C2H2), ethyl alcohol (C2H5OH), benzene (C6H6) and acetic acid (CH3COOH). Benzene is a basic structural unit of organic molecules that form ring structures. Chemists refer to these as "aromatic" compounds because of their distinctive odors.