Fractional Distillation of Crude Oil

This lesson explains how crude oil is separated into useful fractions by fractional distillation, as required by the Edexcel GCSE Chemistry specification (1CH0). You need to understand the process, the temperature gradient, why different fractions condense at different heights, and the names and uses of the main fractions.

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Why Separate Crude Oil?

Crude oil in its raw form is not very useful. It is a mixture of hundreds of different hydrocarbons with different chain lengths. To make useful products (fuels, plastics, chemicals), the oil must be separated into groups of hydrocarbons with similar chain lengths and boiling points. Each group is called a fraction.

Because crude oil is a mixture (not a compound), it can be separated by physical methods — specifically, fractional distillation.

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The Process of Fractional Distillation

Fractional distillation takes place in a fractionating column — a tall tower at an oil refinery.

Steps

1. Heating: Crude oil is heated in a furnace to about 350–400 °C. This vaporises most of the hydrocarbons, turning them into gas.
2. Entering the column: The hot vapour mixture enters the fractionating column near the bottom.
3. Temperature gradient: The column is hot at the bottom and cool at the top. There is a continuous temperature gradient from roughly 350 °C at the base to about 25 °C at the top.
4. Rising and condensing: The vapours rise up the column. As they reach a level where the temperature is below their boiling point, they condense back into a liquid and are collected.
5. Collection: Different fractions are drawn off at different heights via pipes (called side streams).

Key Principle

• Hydrocarbons with short chains (low boiling points) rise to the top of the column before they condense.
• Hydrocarbons with long chains (high boiling points) condense near the bottom of the column.

Exam Tip: Each fraction is still a mixture of hydrocarbons — but they all have similar chain lengths and similar boiling points. No fraction is a single pure compound.

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Diagram of the Fractionating Column

graph TB
    subgraph Fractionating Column
        A["🔥 Heated crude oil vapour enters (≈ 350 °C)"] --> B["Bitumen<br/>Long chains, very high b.p.<br/>Collected at bottom"]
        A --> C["Fuel oil / Heavy oil<br/>Used in ships, power stations"]
        A --> D["Diesel oil<br/>Used in diesel engines"]
        A --> E["Kerosene<br/>Jet fuel"]
        A --> F["Petrol (gasoline)<br/>Car fuel"]
        A --> G["Refinery gases (LPG)<br/>Short chains, low b.p.<br/>Collected at top"]
    end

    style A fill:#ff6b35,color:#fff
    style B fill:#8B4513,color:#fff
    style C fill:#A0522D,color:#fff
    style D fill:#CD853F,color:#fff
    style E fill:#DAA520,color:#fff
    style F fill:#FFD700,color:#000
    style G fill:#87CEEB,color:#000

Temperature increases from top to bottom. The gases collected at the top have the lowest boiling points; the bitumen collected at the bottom has the highest boiling point.

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The Main Fractions

Fraction	Approximate chain length	Boiling point range	State at room temperature	Main use(s)
Refinery gases (LPG)	C₁–C₄	Below 25 °C	Gas	Bottled gas (heating, cooking)
Petrol (gasoline)	C₅–C₁₀	25–75 °C	Liquid	Car fuel
Kerosene	C₁₀–C₁₆	150–240 °C	Liquid	Jet fuel
Diesel oil	C₁₄–C₂₀	220–350 °C	Liquid	Diesel engines, trains
Fuel oil / Heavy oil	C₂₀–C₅₀	350–600 °C	Thick liquid	Ships, power stations
Bitumen	C₅₀+	Above 600 °C	Solid / very thick	Roads, roofing

Exam Tip: You do not need to memorise exact chain lengths for each fraction, but you must know the order (from gases at the top to bitumen at the bottom) and be able to describe the trend — shorter chains have lower boiling points and are collected higher up the column.

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Why Does Fractional Distillation Work?

The molecules in crude oil are held together by intermolecular forces (forces between molecules). The strength of these forces depends on the size of the molecule:

• Longer hydrocarbon chains have stronger intermolecular forces because there are more points of contact between neighbouring molecules.
• Stronger intermolecular forces mean more energy is needed to separate the molecules and turn them into a gas.
• Therefore, longer chains have higher boiling points.

Because different-length hydrocarbons boil at different temperatures, they condense at different heights in the column — this is why the mixture can be separated.

Chain length	Intermolecular forces	Boiling point	Condensation point in column
Short	Weak	Low	Near the top (cool)
Long	Strong	High	Near the bottom (hot)

Exam Tip: Fractional distillation works because different hydrocarbons have different boiling points. The separation is a physical process — no chemical bonds are broken.

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Fractions Are Mixtures

Each fraction collected from the column is itself a mixture of hydrocarbons with similar boiling points. For example, the petrol fraction contains a range of hydrocarbons from about C₅ to C₁₀. These can be further separated or processed if needed.

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Worked Example — Interpreting the Column

Question: Explain why kerosene is collected at a higher point in the fractionating column than diesel oil.

Answer:

1. Kerosene has shorter hydrocarbon chains than diesel oil.
2. Shorter chains have weaker intermolecular forces.
3. Less energy is needed to overcome these forces, so kerosene has a lower boiling point than diesel.
4. In the fractionating column, the temperature is lower higher up. Kerosene vapour condenses at a higher (cooler) point than diesel, so it is collected further up the column.

Exam Tip: "Explain" questions about fractional distillation almost always require you to link chain length → intermolecular forces → boiling point → position in column. Practise writing this chain of reasoning.

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Why Refining Matters

Crude oil is one of the most important raw materials in the modern world. The products of fractional distillation provide: