GCSE Chemistry Rock and Metals – Drude Oil

C1a 3.1 Crude Oil

  • Crude oil is made of many different chemical compounds
  • Crude oil contains hydrocarbons, which are compounds of carbon and hydrogen atoms only
  • Hydrocarbons which are saturated are called alkanes. Being saturated means they have as much hydrogen in their molecules as possible
  • The general formula for alkane molecules is:


  • This means that for every carbon atom there is – n – there are twice as many hydrogen atoms plus two more– 2n+2


    C1a 3.2 Fractional Distillation

  • We can separate crude oil using fractional distillation
  • The chains of hydrocarbons vary in size, and differently- sized hydrocarbon chains have different properties.
  • We separate crude oil into fractions, which are groups of hydrocarbons with similar properties
  • Each fraction of hydrocarbon molecules contains hydrocarbons with similar amounts of carbon atoms, each of these fractions boil at different temperatures because of the number of carbon atoms per molecule
  • Crude oil is fed into the bottom of the fractioning column as hot vapour. The tower is kept at high temperatures at the bottom and is cooler at the top, so the column decreases in temperature as you go up the column
  • These different gases condense when they reach their boiling points, and the different fractions are collected at different levels
    – Hydrocarbons with the smallest molecules have the lowest boiling points, and are collected at the top of the tower. At the bottom, fractions have high boiling points, they cool to form thick liquids or solids at room temperature

    C1a 3.3 Cleaner Fuels

    • Burning hydrocarbons releases substances such as carbon dioxide into the atmosphere
    • Some substances dissolve in droplets of water and fall as acid rain
    • Carbon dioxide released by burning fuels is a greenhouse gas which reduces the rate at which energy is lost from the surface of the Earth by radiation
    • We can use cleaner fuels which don’t release any or as many greenhouse gases, e.g. gasohol