Fuel oil in a beaker. Fuel oil is a product of the refining of crude oil. Crude oil is a mixture of hydrocarbons that is heated to separate it into parts called fractions. The lighter hydrocarbon fractions boil off and leave the heavier fractions behind. Fuel oil is one of the heaviest fractions, containing hydrocarbon chains with 20-70 carbon atoms. It boils in the range 370-600 degrees Celsius. Fuel oil is used as an industrial fuel, a ship fuel, and to make other oil products. To compare the colour and viscosity of crude oil and its fractions, see T110/483-491 and T110/492-501.