The base unit of all fossil fuels, including crude oil, are hydrocarbon molecules composed of hydrogen (H) and carbon (C) atoms.

However, not all hydrocarbons are the same in shape, size and weight - some are short chains such as methane (CH₄) as is the case for natural gas, and some form rings such as benzene (C₆H₆). The gasoline that we use in our vehicles is really a collection of a specific range of hydrocarbon sizes (around C₄H₁₀ to C₁₃H₂₈), which are smaller than the hydrocarbons that make up diesel (around C₁₄H₃₀ to C₁₇H₃₆).

Differences in the size, shape and weight of hydrocarbons mean that they have different chemical and physical reaction properties, including the temperature at which they boil. Generally, the larger the hydrocarbon, the heavier it is and the more energy it will take to boil. Using these differences in boiling points and weights, fossil fuels can initially be separated into their various parts within distillation towers