Fractional distillation and petrochemicals: Many of the fuels on which we depend for our modern lifestyle, such as petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases, are produced from crude oil.

The idea

Crude oil is a finite mixture of hydrocarbons. It is separated by fractional distillation because the fractions have different boiling points. The column is hot at the bottom and cooler at the top. Each fraction condenses at a different height and is tapped off. Petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases are examples of these fractions. They are fuels for cars, lorries, aircraft, ships and heating. Smaller molecules have lower boiling points, are more volatile, flow more easily and burn more cleanly. Larger molecules have higher boiling points, are more viscous and burn with a smoky flame. This explains why different fractions suit different uses.

What earns the marks

• Crude oil is a mixture of hydrocarbons that must be separated into fractions with different boiling point ranges.
• Fractional distillation uses a fractionating column with a temperature gradient: hot at the bottom, cooler at the top.
• Each fraction condenses at a different height because the hydrocarbons in it have a specific boiling point range.
• Petrol, diesel oil, kerosene, heavy fuel oil and liquefied petroleum gases are named fractions obtained from crude oil.
• The fractions are used as fuels because they release energy when burned, supporting transport, heating and industry.
• Smaller hydrocarbon molecules have lower boiling points, are more volatile and ignite more easily than larger molecules.

Common mistakes and how to put them right

• Thinking crude oil is a single compound rather than a mixture.

  Put it right: Crude oil is a mixture of many hydrocarbons with different boiling points.

• Believing fractional distillation chemically changes the hydrocarbons.

  Put it right: It is a physical separation process; the molecules themselves are unchanged.

• Confusing the order of fractions in the column.

  Put it right: Smaller molecules with lower boiling points leave near the top, while larger molecules with higher boiling points leave near the bottom.

Exam tips

• Link each named fuel to its use, for example kerosene for aircraft fuel and heavy fuel oil for ships, to show understanding of why the fraction is suitable.
• When describing the column, state clearly that it is hot at the bottom and cooler at the top, and that fractions condense at different heights.
• Use comparative language such as lower boiling point, more volatile and more viscous to explain trends rather than simply listing fractions.