Fractional Distillation – IGCSE Chemistry Definition

Fractional distillation is tested in two contexts at IGCSE: as a laboratory technique (separating miscible liquids) and as the industrial process for refining crude oil. Both appear regularly on Papers 2, 4, and 6.

The 0620 definition

Fractional distillation is a separation technique used to separate a mixture of miscible liquids with different boiling points, using a fractionating column.

Laboratory fractional distillation

Apparatus

• Round-bottomed flask containing the liquid mixture
• Fractionating column (packed with glass beads to increase surface area)
• Thermometer at the top of the column
• Condenser (water-cooled)
• Collection flask

How it works

1. The mixture is heated in the flask
2. All liquids vaporise
3. Vapours rise up the fractionating column
4. The column is cooler at the top
5. Liquids with higher boiling points condense and drip back down
6. The liquid with the lowest boiling point reaches the top first
7. Its vapour passes into the condenser, cools, and is collected as the distillate
8. The temperature rises and the next liquid can then be collected

Fractional distillation of crude oil

Crude oil is a mixture of hydrocarbons. It is separated in a fractionating column (tower):

Fraction	Boiling range	Chain length	Use
Refinery gas	Below 25 °C	C₁−C₄	Fuel gas, LPG
Petrol/gasoline	25–75 °C	C₅−C₁₀	Vehicle fuel
Naphtha	75–150 °C	C₈−C₁₂	Chemical feedstock
Kerosene	150–250 °C	C₁₁−C₁₆	Jet fuel
Diesel	250–350 °C	C₁₅−C₂₀	Vehicle fuel
Fuel oil	350–500 °C	C₂₀−C₃₅	Fuel for ships, power stations
Bitumen	Above 500 °C	C₃₅+	Roads, roofing

Short-chain fractions: lower boiling points, more volatile, less viscous, more flammable, lighter colour.

Worked exam question

Crude oil is separated by fractional distillation. (a) Explain why crude oil can be separated in this way. [1] (b) State where in the column the fraction with the lowest boiling point is collected. [1] (c) State one property that changes as you go up the column. [1]

Mark scheme

(a) The hydrocarbons in crude oil have different boiling points [1]

(b) At the top [1]

(c) Boiling point decreases / viscosity decreases / flammability increases / chain length decreases [1] (any one)

Common exam mistakes

• Saying fractional distillation “breaks apart” molecules — it only separates them physically. Cracking breaks molecules.
• Confusing which fractions come from the top vs bottom — short chains (low boiling point) come from the top; long chains from the bottom.
• Forgetting the fractionating column — without it, you are describing simple distillation, not fractional.