Petroleum

Petroleum, including liquid oil and natural gas, consists of substances known as hydrocarbons. Hydrocarbons, as their name suggests, comprise hydrogen and carbon, with small amounts of impurities such as nitrogen, oxygen, and sulfur. The molecules of hydrocarbons can be as simple as that of methane, which consists of a carbon atom surrounded by 4 hydrogen atoms, abbreviated as CH₄. More complex hydrocarbons, such as naphthenes, include rings of hydrogen and carbon atoms linked together. Differences in the number of hydrogen and carbon atoms in molecules as well as their molecular structure (carbon atoms arranged in a ring structure, chain, or tetrahedron, for example) produce numerous types of petroleum.

Different types of petroleum can be used in different ways. Jet fuel differs from the gasoline that automobiles consume, for example. Refineries separate different petroleum products by heating petroleum to the point that heavy hydrocarbon molecules separate from lighter hydrocarbons so that each product can be used for a specific purpose. Refining reduces the waste associated with using limited supplies of more expensive petroleum products in cases in which a cheaper, more plentiful type of petroleum would suffice. Thus, tar or asphalt, the dense, nearly solid hydrocarbons, can be used for road surfaces and roofing materials, waxy substances called paraffins can be used to make candles and other products, and less dense, liquid hydrocarbons can be used for engine fuels.