Amorphous carbon is a deep black powder that occurs in nature as a component of coal. It may be obtained artificially from almost any organic substance by heating the substance to very high temperatures without air. Using this method, coke is produced from coal, and charcoal is produced from wood. Amorphous carbon is the most reactive form of carbon. Because amorphous carbon burns easily in air, it is used as a combustion fuel.
Inorganic compounds are binary compounds of carbon with metals or metal carbides.
Carbon compounds containing nonmetals are usually gases or liquids with low boiling points. Carbon monoxide, a gas, is odorless, colorless, and tasteless. Carbon dioxide is a colorless, almost odorless gas that is formed by the combustion of carbon. It is a product that results from respiration in most living organisms and is used by plants as a source of carbon. Frozen carbon dioxide, known as dry ice, is used as a refrigerant.