Petroleum is a viscous liquid containing a variety of different hydrocarbons and organic material that produces significant amounts of energy when burned. In its primordial state, it is difficult to work with, but raw petroleum can be refined into gasoline, kerosene and a variety of other fuels and products.
Petroleum forms when decaying organic materials are trapped in layers of sediment. Decomposers remove most of the oxygen, nitrogen and phosphorus, leaving mostly carbon and hydrogen. Over time, heat and pressure work on the remains to fuse the carbon and hydrogen into chains called hydrocarbons. These molecules store energy in their bonds that can be released when the petroleum is burned.
When discovered, petroleum is classified according to its composition. "Light" petroleum is less dense, while "heavy" petroleum is much thicker and requires more refining to produce gasoline and other light products. "Sweet" petroleum has a low sulfur content, while "sour" deposits have an abundance of sulfur. This can affect the pollutants released when the petroleum is burned, creating a dangerous gas called sulfur dioxide that can contribute to acid rain.
Under ideal circumstances, a hydrocarbon fuel would burn completely, producing only carbon dioxide and water vapor as byproducts. However, impurities and fluctuating temperatures can cause incomplete combustion, releasing unburned hydrocarbons and other potentially toxic gases into the air. Modern automobiles use catalytic converters containing reactive metals to reduce potentially harmful emissions.