Petrochemicals are derived from a variety of sources, including coal and natural gas, though the majority of the petrochemicals we use today are made from petroleum. Understanding how petroleum—also known as crude oil—is transformed into petrochemicals begins with a basic understanding of petroleum itself. Petroleum is found deep in the Earth's crust, beneath land and ocean alike, and it is formed from decaying plant and animal matter. While not considered a chemical compound in and of itself, petroleum is instead composed of scores of individual chemical compounds, all of which contain carbon and hydrogen atoms, which are referred to as hydrocarbons. These hydrocarbons can either be simple or quite complex, ranging from a few atoms to nearly 100 atoms. Once petroleum is extracted from the ...