A fuel is any compound that has stored energy. This energy is captured in chemical bonds through processes such as photosynthesis and respiration. Energy is released during oxidation. The most common form of oxidation is the direct reaction of a fuel with oxygen through combustion. Wood, gasoline, coal, and any number of other fuels have energy-rich chemical bonds created using the energy from the Sun, which is released when the fuel is burned (i.e., the release of chemical energy). Chemical fuels or the fossil fuels are useful reserve of fuels and are therefore used extensively to satisfy the demands of an energy-dependent civilization.

Fossil fuels are principally hydrocarbons with minor impurities. They are so named because they originate from the decayed and fossilized remains of plants and animals that lived millions of years ago.

Fossil fuels can be separated into three...