Crude Oil
Crude oil contains mainly saturated hydrocarbons called alkanes.

Crude Oil

Crude oil is nicknamed 'black gold' as it has made many people very rich. It is formed from the remains of dead sea creatures and plants that sink to the bottom of the oceans where they decay anaerobically as they become buried in the sea bed mud. As the mud turns to rock and becomes buried by newer sediments, the oil, being a liquid, will rise and can become trapped underground by an impermeable rock layer. This trapped oil can be released by drilling through the impermeable rock. This GCSE Chemistry quiz is all about the compounds that can be extracted from it.

Some oil does not get trapped and seeps out at the surface of the Earth, for example at the 'Fountains of Pitch' in Iraq. At the surface, most of the volatile chemicals evaporate leaving behind a thick tar-like substance called either pitch or asphalt. Crude oil was first used in this form about 6000 years ago by ancient civilisations of the Middle East for waterproofing boats and pottery as well as to stick building stones together. The Chinese are thought to have been the first to have drilled for oil almost 2000 years ago.

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The modern petroleum industry began in the nineteenth century. Scottish chemist James Young managed to successfully distill a light oil that could be used in oil lamps from a seepage of petroleum from a mine in Derbyshire. He was also the first person to be able to obtain a substance like crude oil from coal and set up what is regarded as the first commercial oil refinery with two business partners in Glasgow. Chemists in Eastern Europe were also distilling crude oil at about the same time as Young but it is generally accepted that this industry really began when an American called Edwin Drake drilled his oil well in 1859 in Pennsylvania in the USA. There were earlier wells but this was the first to be mechanically drilled.

Young's successful distillation relied on the fact that the compounds in crude oil have different boiling points. Crude oil is a mixture of hydrocarbons from the alkane family which can be extracted. As you heat up the mixture, the alkanes with the lowest boiling points will start to evaporate. Their vapour can be collected and condensed - you may have seen this demonstrated at school. As each alkane evaporates off, the temperature will rise and the next will boil and so on until you are left with the alkane with the highest boiling point.

The industrial process is slightly different. The crude oil is completely vapourised and introduced into a tall steel vessel called a fractionating column. Inside this, there is a temperature gradient; the base is the hottest and the top is the coolest. As the vapours rise, they cool. When they reach the part of the column that is at the temerature equal to their boiling point, they condense. There are collecting trays at different points within the column that collect the condensed fractions of oil. Each of the fractions of crude oil still contains a mixture of alkanes, but with a much smaller range of carbon chain lengths than the original oil.

Try this quiz and see how well you understand the compounds that can be extracted from crude oil.

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  1. Crude oil contains mainly saturated hydrocarbons called...
    A saturated hydrocarbon contains only single bonds
  2. Hydrocarbons with the smallest molecules have the...
    Smaller molecules have smaller inter-molecular forces to hold them together
  3. Which of these compounds has the highest boiling point?
    The more carbon atoms there are in an alkane, the larger the molecule and the higher the boiling point
  4. Which of these compounds catches fire most easily?
    Alkanes with smaller molecules are far more volatile than large ones. It is the vapour from a flammable liquid that burns so the more volatile chemicals from crude oil are easier to ignite
  5. Fractional distillation uses which physical property to separate the substances in crude oil?
    Crude oil is a mixture of hundreds of alkanes and each has a different boiling point
  6. Which of the following is a saturated hydrocarbon that could be found in diesel oil?
    Diesel cars give better fuel consumption figures than petrol cars because diesel fuel molecules are larger and therefore release more energy than petrol molecules which only have about 8 carbon atoms
  7. Petrol...
    It contains alkanes with fewer carbon atoms than diesel
  8. Crude oil is fed into the fractionating column...
    It is heated to a temperature in excess of 300oC
  9. Which of these compounds is collected at the top of the fractionating column?
    This is propane which is a gas at room temperature
  10. Which of the following is the thickest liquid?
    Viscosity of the alkanes increases as the molecule gets bigger

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