Cracking is the thermal decomposition of long-chain alkanes (which are in low demand) into shorter, more useful alkanes and highly reactive alkenes. It requires high temperatures and a catalyst (like aluminium oxide).

What are the characteristics of fractions collected at the bottom of the column?

Fractions collected at the bottom (like Bitumen) have long carbon chains. They flow very slowly (high viscosity), are difficult to ignite (low flammability), have high boiling points, and burn with a smoky flame.

O-Level Chemistry: Fractional Distillation of Crude Oil

Q: What property allows fractional distillation to separate crude oil?

Different boiling points. Crude oil is a mixture of hydrocarbons. Smaller hydrocarbons have lower boiling points and rise to the top of the fractionating column. Larger hydrocarbons have higher boiling points and condense near the hotter bottom.

Q: What is the order of the fractions from top to bottom?

Refinery Gas (top), Gasoline/Petrol, Naphtha, Kerosene/Paraffin, Diesel, Fuel Oil, and Bitumen (bottom). Remember the mnemonic: Ripe Gorillas Never Keep Dirty Food Bags.

$A fractionating column separating crude oil into different hydrocarbon chains based on boiling points.$

How does a fractionating column separate crude oil?

A fractionating column relies on a temperature gradient — it is hot at the bottom and cool at the top. Crude oil is heated into vapour and enters the column. Hydrocarbons with long chains (high boiling points) condense quickly at the hot bottom. Short chains (low boiling points) rise all the way to the cool top before condensing.

1. The Principle of Separation
2. The 7 Fractions You Must Memorise
3. Group Trends Down the Column
4. Cracking: Solving the Demand Imbalance
5. Frequently Asked Questions

The industrial separation of crude oil is guaranteed to appear on O-Level Chemistry Paper 2. Examiners will test your memory of the fraction names, their uses, and the physical trends as carbon chains get longer. This guide from our Ultimate O-Level Chemistry Guide provides the exact mnemonics you need to lock down these marks.

1. The Principle of Separation

Crude oil is a useless black sludge. It is a mixture of hundreds of different hydrocarbons (molecules containing ONLY Hydrogen and Carbon). To make it useful, we must separate it.

The separation technique is Fractional Distillation, and it relies entirely on one physical property: Boiling Point.

Step 1: The crude oil is heated in a furnace until most of it vaporises.
Step 2: The vapours enter the fractionating column. There is a temperature gradient (hot at the bottom, cool at the top).
Step 3: Vapours rise up the column until they reach a compartment where the temperature is lower than their boiling point. There, they condense into liquids and are tapped off.

2. The 7 Fractions You Must Memorise

From TOP (coolest) to BOTTOM (hottest), you need to know the names of the fractions and their primary uses.

Fraction	Primary Use (CAIE Accepted Answers)
Refinery Gas	Bottled gas for heating and cooking
Gasoline / Petrol	Fuel for cars
Naphtha	Chemical feedstock (making plastics)
Kerosene / Paraffin	Jet engine fuel
Diesel Oil	Fuel for heavy vehicles (trucks, trains)
Fuel Oil	Fuel for ships and home heating systems
Bitumen	Making roads and roof surfacing

💡 Tutor's Tip

Mnemonic to remember the order from top to bottom: Ripe Gorillas Never Keep Dirty Food Bags.

3. Group Trends Down the Column

As you move DOWN the fractionating column (from Refinery Gas to Bitumen), the carbon chains get much longer. This causes consistent physical changes.

Boiling Point INCREASES: Longer chains have stronger intermolecular forces pulling them together. It takes more heat energy to separate them.
Viscosity INCREASES: The liquid gets thicker and flows much slower. (Bitumen is basically solid tar).
Flammability DECREASES: Longer chains are much harder to light on fire.
Volatility DECREASES: Longer chains do not turn into a gas easily at room temperature.

4. Cracking: Solving the Demand Imbalance

Crude oil gives us lots of long-chain fractions (like Bitumen and Fuel Oil), but nobody really wants them. The world runs on short-chain fractions (like Petrol/Gasoline). Supply doesn't match demand.

To fix this, oil refineries use Cracking.

Thermal Cracking Rules:

Process: Breaking down long, useless alkanes into short, useful alkanes + highly reactive alkenes.
Conditions: Very high temperature (600°C) and a catalyst (Aluminium Oxide or Silicon Dioxide).
Equation rule: The total number of Carbons and Hydrogens on the left MUST equal the right. E.g., C₁₀H₂₂ → C₈H₁₈ + C₂H₄

📋 From the Desk of Dr. Aisha Rahman

If an exam question asks "What is the economic importance of cracking?", you need two points. First point: It produces petrol to meet the massive global demand for car fuel. Second point: It produces alkenes (like ethene), which are the essential building blocks for making plastics (polymers). Never forget the plastics!

Frequently Asked Questions

What property allows fractional distillation to separate crude oil?▼

Different boiling points. Small molecules boil at low temperatures; large molecules boil at high temperatures.

What is the order of the fractions from top to bottom?▼

Refinery Gas, Gasoline/Petrol, Naphtha, Kerosene, Diesel, Fuel Oil, Bitumen. Use the mnemonic Ripe Gorillas Never Keep Dirty Food Bags.

What is cracking?▼

Thermal decomposition of heavy, long-chain alkanes into smaller, more valuable alkanes (like petrol) and reactive alkenes (for making plastics).

What are the characteristics of fractions at the bottom of the column?▼

They have very long carbon chains. Because of this, they have high viscosity (thick), low flammability (hard to burn), and very high boiling points.

Stop Guessing, Start Scoring

Get instant access to 500+ CAIE-aligned practice questions, worked solutions, and AI-powered mock exams across all O-Level subjects.

Access the Study Portal →Prepare with the Oracle Engine

Fractional Distillation: The Crude Oil Blueprint