Crude oil is a mixture of a very large number of compounds.

The idea

Crude oil is a naturally occurring, dark, viscous liquid found in porous rocks underground. It is not a single substance but a mixture of a very large number of different compounds, mostly hydrocarbons, which are not chemically combined and can be separated by physical means such as fractional distillation. Because the components have different boiling points, they can be separated without chemical reaction. For example, when crude oil is heated, the compounds with lower boiling points rise as vapour and condense at different heights in a fractionating column, producing fractions such as petrol and diesel. The mixture has no fixed boiling point and its composition varies depending on its source.

What earns the marks

• Crude oil is a mixture, not a compound or a pure substance.
• A mixture contains two or more substances that are not chemically combined.
• Crude oil contains a very large number of different compounds.
• The compounds in crude oil can be separated by physical processes such as fractional distillation.
• Because the components have different boiling points, they can be separated without chemical reaction.
• The composition of crude oil varies depending on where it is extracted.

Common mistakes and how to put them right

• Describing crude oil as a compound or element.

  Put it right by stating it is a mixture of many compounds.

• Stating that crude oil has a fixed boiling point.

  Put it right by explaining that a mixture boils over a range of temperatures.

• Claiming that fractional distillation involves chemical reactions.

  Put it right by stating it is a physical separation based on boiling points.

Exam tips

• Define 'mixture' clearly before applying it to crude oil.
• Link the separation method to the physical property of boiling point.
• Use the phrase 'very large number of compounds' to match the specification wording.