fossil fuels are made up of compounds called
Because fossil fuels form from the remains of plants and animals, all fossil fuels contain carbon, too. Most of the carbon in fossil fuels exists as hydrocarbons, which are hydrogen-carbon compounds. Fossil fuels may exist as liquids, gases, or solids.Liquid Fossil Fuels: Petroleum A liquid mixture of complex hydrocarbon compounds is called petroleum.oal is a fossil fuel that forms underground from partially decomposed plant material. Coal was once the major source of energy in the United States. When fossil fuels, including coal, are burned, most of the energy released is heat.