It is also known as petroleum .
Crude oils vary in color , from clear to tar-black, and in viscosity , from water to almost solid.
Crude oils are such a useful starting point for so many different substances because they contain hydrocarbons . Hydrocarbons are molecules that contain hydrogen and carbon and come in various lengths and structures, from straight chains to branching chains to rings. Hydrocarbons can take on many different forms. Longer chains with 5 or more carbons are liquids. Very long chains are solids like wax or tar.