Crude Oil, Fractional Distillation and Fuels
Crude oil is a mixture of hydrocarbons, separated by fractional distillation into fractions with different uses. The trends down the column follow one rule: chain length.
1. What crude oil is
Crude oil is a MIXTURE of HYDROCARBONS — compounds containing hydrogen and carbon ONLY.
A hydrocarbon contains ONLY hydrogen and carbon. If a compound contains oxygen (like ethanol) or any other element, it is not a hydrocarbon. Uncertainty about identifying hydrocarbons was recorded.
Crude oil is a finite, non-renewable fossil fuel, formed over millions of years from the remains of marine organisms under heat and pressure.
2. Fractional distillation
The fractions are separated by their different BOILING POINTS, which depend on chain length.
How it works:
- Crude oil is heated and vaporised
- The vapour enters the fractionating column, which is HOT at the BOTTOM and COOLER at the TOP
- Each vapour rises until it reaches a temperature below its boiling point, where it condenses and is collected
- Short chains have low boiling points, so they rise high before condensing; long chains condense near the bottom
The result is a temperature gradient down the column — this is the single most examined mechanism on the page.
The fractions, from the top (shortest chains) down:
| Fraction | Uses |
|---|---|
| Refinery gases | bottled gas for heating and cooking |
| Petrol (gasoline) | fuel for cars |
| Naphtha | feedstock for chemicals and plastics |
| Kerosene / paraffin | fuel for aircraft |
| Diesel oil | fuel for lorries and some cars |
| Fuel oil | fuel for ships and heating |
| Bitumen | road surfacing and roofing |
Fractional distillation does NOT give fixed percentages — the proportions depend on the crude oil source, which is one reason cracking is needed. Uncertainty about this was recorded.
3. The trends down the column
As the chains get LONGER (going down the column):
| Property | Trend |
|---|---|
| Boiling point | INCREASES |
| Viscosity (thickness) | INCREASES |
| Volatility (ease of evaporating) | DECREASES |
| Flammability (ease of ignition) | DECREASES |
| Colour | darker |
Why: longer molecules have stronger intermolecular forces, so more energy is needed to separate them.
Going DOWN the column, fractions are LESS volatile and LESS flammable. A recorded error had it the other way round — a student thought going down meant more flammable. It is the short chains at the top that ignite most easily, which is why petrol is a car fuel and bitumen is a road surface.
Volatility means how easily a liquid evaporates. Uncertainty about the volatility of longer chains was recorded — longer chain = less volatile.
4. Cracking
CRACKING breaks long-chain hydrocarbons into shorter, more useful ones — producing alkanes and ALKENES.
Conditions:
Catalytic cracking: a catalyst (silica or alumina) at about 600–700 °C Or: high temperature and steam
State the conditions — temperature AND catalyst. Tutors flagged this directly, and uncertainty about the conditions was recorded twice.
Why it is done:
1. Supply and demand: there is more demand for short-chain fractions (petrol) than crude oil naturally provides, and a surplus of long chains. 2. Alkenes are produced, which are needed to make polymers and ethanol.
Explain the use of the short-chain products for full marks — tutors flagged this. Say they are used as fuels and to make plastics.
Balancing a cracking equation:
The carbons and hydrogens must balance across the products.
Example: C₁₀H₂₂ → C₈H₁₈ + C₂H₄
Check: C: 10 = 8 + 2 ✓ H: 22 = 18 + 4 ✓
An ALKENE is always among the products — that is what makes cracking valuable. Difficulty balancing cracking equations, including for decane, was recorded; work out the second product by subtracting the first from the original.
5. Combustion
COMPLETE combustion (plenty of oxygen): hydrocarbon + O₂ → carbon dioxide + water INCOMPLETE combustion (limited oxygen): → carbon monoxide and/or carbon (soot) + water
Problems with fuels:
| Pollutant | Source | Problem |
|---|---|---|
| Carbon dioxide | complete combustion | greenhouse gas → climate change |
| Carbon monoxide | incomplete combustion | toxic — binds to haemoglobin |
| Sulfur dioxide | sulfur impurities in the fuel | ACID RAIN |
| Oxides of nitrogen | nitrogen and oxygen reacting in hot engines | acid rain, photochemical smog |
| Soot (carbon) | incomplete combustion | breathing problems, blackens buildings |
Carbon monoxide is the toxic one; carbon dioxide is the greenhouse gas. Keep them apart.
6. Mistakes that cost marks
Saying a compound with oxygen is a hydrocarbon.
Getting the column trends backwards.
Saying longer chains are more volatile or more flammable.
Not stating both cracking conditions.
Unbalanced cracking equations.
Forgetting an alkene among the cracking products.
Confusing the pollutants and their effects.
Saying fractional distillation gives fixed proportions.
Frequently asked questions
What is crude oil? A mixture of hydrocarbons — a finite fossil fuel.
What is a hydrocarbon? A compound of hydrogen and carbon only.
How are the fractions separated? By fractional distillation, using their different boiling points.
Where are short chains collected? Near the top, where it is cooler — they have low boiling points.
What happens to boiling point as chains get longer? It increases, as do viscosity — while volatility and flammability decrease.
What is cracking? Breaking long chains into shorter alkanes and alkenes.
What conditions does cracking need? A catalyst at about 600–700 °C, or high temperature with steam.
Why is cracking done? To meet demand for short chains and to produce alkenes for polymers.
What does incomplete combustion produce? Carbon monoxide and/or soot, plus water.
Which pollutant causes acid rain? Sulfur dioxide (and oxides of nitrogen).
Quick revision checklist
- I know crude oil is a mixture of hydrocarbons
- I can define a hydrocarbon
- I can explain fractional distillation and the temperature gradient
- I know the fractions in order and their uses
- I know all the trends with chain length
- I know longer chains are less volatile and flammable
- I know what cracking is and both its conditions
- I can balance a cracking equation
- I know why cracking is done — demand and alkenes
- I know complete and incomplete combustion products
- I can name the pollutants and their effects
These notes cover crude oil, fractional distillation and fuels in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for reaction conditions in your own exam series.
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