FRACTIONAL DISTILLATION separates crude oil into FRACTIONS — groups of hydrocarbons with similar boiling points and chain lengths.
The process:
1. Crude oil is HEATED to ~350°C — most of it vaporises.
2. Vapours rise up a tall FRACTIONATING COLUMN which is HOT at the bottom and COOL at the top.
3. As vapours rise, they COOL DOWN.
4. Each fraction CONDENSES (turns back to liquid) at its boiling point and is collected at the appropriate level.
5. Fractions with HIGHER boiling points (longer chains) condense LOWER in the column.
6. Fractions with LOWER boiling points (shorter chains) rise higher and condense near the TOP.
Heavy residue (bitumen) doesn't vaporise and remains at the bottom.
Refinery gases (shortest chains: methane, ethane, propane, butane) rise to the very top and leave as gases.
The Main Fractions and Their Uses
From top (lowest boiling point) to bottom (highest boiling point):
REFINERY GASES (C₁–C₄): bp below 25°C
Shortest chains — methane, propane, butane.
Uses: LPG (cooking and heating), camping gas.
PETROL/GASOLINE (C₅–C₁₀): bp 25–75°C
Uses: fuel for cars.
NAPHTHA (C₅–C₁₀): bp 75–120°C
Uses: feedstock for making chemicals and plastics.
KEROSENE/JET FUEL (C₁₀–C₁₆): bp 150–250°C
Uses: jet aircraft fuel, heating.
DIESEL/GAS OIL (C₁₅–C₂₅): bp 220–350°C
Uses: fuel for lorries, trains, some cars.
FUEL OIL (C₂₀–C₇₀): bp >350°C
Uses: large ships, power stations.
BITUMEN (C₇₀+): doesn't vaporise
Uses: road surfacing, roofing.
Properties and Trends in Fractions
As chain length INCREASES going from top to bottom of the fractionating column:
BOILING POINT INCREASES:
Longer chains → stronger London dispersion forces (intermolecular) between molecules → more energy to separate → higher boiling point.
VISCOSITY INCREASES (gets thicker/runnier):
Longer chains → more tangled molecules → harder to flow.
Refinery gases: very low viscosity (gases). Bitumen: extremely viscous (solid/semi-solid).
FLAMMABILITY DECREASES:
Shorter chain fractions ignite more easily — more volatile (evaporate easily at room temperature).
Longer chains are less volatile and harder to ignite.
COLOUR becomes darker — refinery gases are colourless, bitumen is black.
DEMAND vs SUPPLY:
More petrol and diesel are demanded than naturally occur in crude oil.
CRACKING converts larger, less useful fractions into smaller, more useful ones (especially petrol and ethene for plastics).
⚠️ Common Mistake
Fractions with SHORTER chains have LOWER boiling points and collect at the TOP of the fractionating column (cooler). Fractions with LONGER chains have HIGHER boiling points and collect at the BOTTOM (hotter). Students sometimes get this backwards.
📌 Key Note
Fractional distillation: heat crude oil → vapours rise column → cool → condense at boiling point → fractions collected. Short chains: top, low BP, more flammable, less viscous. Long chains: bottom, high BP, less flammable, more viscous. Fractions: gases, petrol, naphtha, kerosene, diesel, fuel oil, bitumen.
🎯 Matching Activity — Match the Fraction to its Property and Use
Match each fraction to its chain length, boiling point and main use. — drag the symbols on the right to match the component names on the left.
Refinery gases
Drop here
Petrol
Drop here
Kerosene
Drop here
Diesel
Drop here
Bitumen
Drop here
Longer chains (C₁₅–C₂₅) — fuel for lorries and trains
Longest chains, doesn't vaporise — road surfacing
Short chains (C₅–C₁₀), low boiling point — fuel for cars
Medium chains (C₁₀–C₁₆) — jet aircraft fuel
Shortest chains (C₁–C₄), lowest boiling point — LPG for cooking and heating
🎯 Test Yourself
Question 1 of 2
1. Where in the fractionating column does petrol collect compared to diesel?
2. Why does bitumen have a much higher boiling point than petrol?
⭐ How Well Do You Understand This Topic?
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