Overview
What you’ll learn
- Describe crude oil as a mixture of hydrocarbons and explain how fractional distillation separates it, naming major fractions and their uses.
- Describe the trends in viscosity, melting/boiling point and flammability down the fractionating column.
- State that alkanes are saturated with general formula CₙH₂ₙ₊₂, and describe their substitution reactions with chlorine in sunlight.
- State that alkenes are unsaturated with general formula CₙH₂ₙ, and describe their addition reactions with hydrogen, steam and halogens.
- Use aqueous bromine to distinguish saturated from unsaturated hydrocarbons.
- Describe cracking as the thermal decomposition of large alkanes into smaller alkanes and alkenes, with equations, and explain its purpose.
In 30 seconds
- 1Crude oil is a mixture of hydrocarbons — compounds of carbon and hydrogen only; fractional distillation sorts it in a tower hot at the bottom, cool at the top.
- 2Down the column fractions get bigger molecules, higher boiling points, thicker (more viscous), less flammable: refinery gas → petrol → naphtha → kerosene → diesel → fuel oil → bitumen.
- 3Alkanes are saturated: single C–C bonds only, general formula CₙH₂ₙ₊₂ — methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀.
- 4Alkanes burn: CH₄ + 2O₂ → CO₂ + 2H₂O (complete); limited air gives CO and soot instead.
- 5Alkanes do substitution with halogens in sunlight: CH₄ + Cl₂ → CH₃Cl + HCl — an H atom is swapped for a halogen.
- 6Alkenes are unsaturated: one C=C double bond, general formula CₙH₂ₙ — ethene C₂H₄, propene C₃H₆.
- 7Alkenes do addition across the double bond: +H₂ (Ni) → alkane; +H₂O (steam) → ethanol; they decolourise orange bromine water instantly (C₂H₄ + Br₂ → C₂H₄Br₂).
- 8Cracking heats big alkanes (with a hot catalyst) to split them: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ — always an alkane + an alkene, because the world needs short chains and alkenes.
Notes · 1 of 5
Crude oil and fractional distillation
A billion-year soup, sorted by boiling point into useful slices.
Crude oil is a mixture of hydrocarbons — no bonds join the different molecules, so they can be separated physically. The fractions have overlapping boiling points: small molecules boil low, big molecules boil high. The fractionating column runs hot at the bottom (about 400 °C) and cools upwards; each fraction condenses on trays at its own level.
| Fraction | Size | Use | Trend |
|---|---|---|---|
| Refinery gas (LPG) | C₁–C₄ | Bottled gas, heating | Smallest, lowest bp, most flammable |
| Petrol | C₅–C₁₀ | Car fuel | Volatile, very flammable |
| Naphtha | C₅–C₁₀ | Chemical feedstock (cracking) | — |
| Kerosene (paraffin) | C₁₀–C₁₆ | Jet fuel | — |
| Diesel oil | C₁₄–C₂₀ | Trucks, buses | Thicker, less flammable |
| Fuel oil | C₂₀–C₅₀ | Ships, power stations | Thick, viscous |
| Bitumen | >C₅₀ | Road surfacing, roofing | Biggest, highest bp, nearly solid |
Notes · 2 of 5
The alkanes: contented hydrocarbons
Single bonds, formula CₙH₂ₙ₊₂, and only fire or sunlight can provoke them.
Alkanes are saturated: every carbon–carbon link is a single bond, so each molecule carries the maximum possible hydrogen. The family formula is CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀. Naming runs on Greek numerals (meth-, eth-, prop-, but-), and each member differs from the next by one –CH₂– unit.
Their great chemical event is combustion: complete burning gives CO₂ and H₂O — CH₄ + 2O₂ → CO₂ + 2H₂O — releasing the heat that makes alkanes fuels. In limited air the same fuel makes carbon monoxide and soot instead — less heat, deadly fumes (see Chapter 8). Their other trick needs sunlight: a halogen atom swaps in for hydrogen — CH₄ + Cl₂ → CH₃Cl + HCl — a substitution reaction, triggered by ultraviolet light. In the dark, chlorine and methane coexist peacefully.
Notes · 3 of 5
The alkenes: hungry hydrocarbons
One double bond changes everything — molecules that add things instead of swapping.
Alkenes carry one carbon–carbon double bond, so they hold less hydrogen than the alkanes: family formula CₙH₂ₙ — ethene C₂H₄, propene C₃H₆. They are unsaturated: the double bond can open up and add atoms across itself, which is why alkenes are the feedstock of the plastics and ethanol industries.
Addition reactions: with hydrogen over a nickel catalyst the double bond closes — C₂H₄ + H₂ → C₂H₆ (hardening vegetable oil into margarine uses exactly this). With steam and a catalyst, C₂H₄ + H₂O → C₂H₅OH — industrial ethanol. With halogens, C₂H₄ + Br₂ → C₂H₄Br₂ — and this is the famous bromine water test.
Notes · 4 of 5
Saturated vs unsaturated at a glance
Same elements, different appetites — one comparison table to rule the test tube.
| Property | Alkanes | Alkenes |
|---|---|---|
| Bonds | All C–C single (saturated) | One C=C double (unsaturated) |
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Bromine water (dark) | No change | Decolourised immediately |
| With chlorine | Substitution, needs UV | Addition, instant |
| Reaction type | Substitution (swap an H) | Addition (open the C=C) |
| Combustion | Burns (fuel) | Burns with a smokier flame |
Notes · 5 of 5
Cracking: splitting big into small
The recycling machine of the refinery — big alkanes in, short alkanes and hungry alkenes out.
Crude oil delivers plenty of big molecules (bitumen, fuel oil) but the world demands short ones — petrol and, above all, alkenes for plastics. Cracking answers the mismatch: pass large-alkane vapour over a hot catalyst (aluminium oxide or silicon dioxide) at high temperature, and the molecule splits: C₁₀H₂₂ → C₈H₁₈ + C₂H₄.
The products are always one smaller alkane plus one alkene — cracking is thermal decomposition, no oxygen involved, nothing burned. Which C–C bond breaks varies, so a mixture of products forms; any split that keeps every atom accounted for is a valid equation. Check: 10 carbons in, 8 + 2 out; 22 hydrogens in, 18 + 4 out. Balance = atoms preserved.
Revise
Key terms
The words examiners expect you to use precisely.
- Hydrocarbon
- A compound of carbon and hydrogen only — the sole ingredient family of crude oil.
- Fractional distillation
- Separating crude oil in a column hot at the bottom and cool at the top, by boiling-point differences.
- Fraction
- A mixture of hydrocarbons with similar boiling points, e.g. petrol, kerosene, diesel.
- Viscosity
- Resistance to flowing; fractions get more viscous down the column as molecules grow.
- Alkane
- A saturated hydrocarbon with only single C–C bonds, formula CₙH₂ₙ₊₂.
- Alkene
- An unsaturated hydrocarbon with one C=C double bond, formula CₙH₂ₙ.
Show 6 more key termsShow fewer key terms
- Saturated
- Holding the maximum hydrogen possible — no double bonds to open.
- Substitution reaction
- A reaction where an atom (e.g. Cl) replaces a hydrogen atom — the alkane halogenation, needing UV light.
- Addition reaction
- A reaction where atoms add across a C=C double bond — the alkene signature.
- Bromine water test
- Orange aqueous bromine decolourised instantly by alkenes (addition), unchanged by alkanes in the dark.
- Cracking
- Thermal decomposition of large alkanes over a hot catalyst into smaller alkanes plus alkenes.
- Hydrogenation
- Adding hydrogen across a C=C bond over a nickel catalyst — hardening oils into margarine.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Calling crude oil a pure compound.
It is thousands of different hydrocarbons mixed, not bonded.
Instead: Crude oil is a MIXTURE — separated physically, not chemically.
Mistake: Saying the fractionating column is hottest at the top.
Low-boiling fractions condense high up — it must be cool there.
Instead: Hot bottom (≈400 °C), cooling upwards; small molecules rise highest.
Show 10 more mistakesShow fewer mistakes
Mistake: Applying CₙH₂ₙ₊₂ to an alkene like propene.
The alkane formula over-counts alkene hydrogens.
Instead: Alkanes CₙH₂ₙ₊₂; alkenes CₙH₂ₙ. Test n first.
Mistake: Writing alkane + chlorine products as CH₃Cl + H₂.
The displaced hydrogen leaves as hydrogen chloride, not hydrogen gas.
Instead: CH₄ + Cl₂ → CH₃Cl + HCl.
Mistake: Forgetting the sunlight/UV condition for alkane halogenation.
In the dark nothing happens — the condition is part of the answer.
Instead: Substitution requires ultraviolet light to break the Cl–Cl bond.
Mistake: Saying bromine water decolourises alkanes quickly too.
Alkanes only substitute slowly, and need UV.
Instead: Instant decolourisation = alkene; no change in the dark = alkane.
Mistake: Writing cracking products as two alkanes.
Cracking always makes one alkane AND one alkene.
Instead: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ — count the carbons and hydrogens to check.
Mistake: Burning oxygen in a cracking equation.
Cracking is thermal decomposition, not combustion.
Instead: No oxygen appears: heat + catalyst only.
Mistake: Ordering fractions by boiling point backwards.
Small molecules boil LOW and condense HIGH.
Instead: Refinery gas at the cool top; bitumen drained at the hot bottom.
Mistake: Hydrogenating ethene without the catalyst condition.
The nickel catalyst (and heat) is part of the answer.
Instead: C₂H₄ + H₂ → C₂H₆ with Ni catalyst, mild heat.
Mistake: Saying alkenes are used mainly as fuels.
Their reactivity makes them too valuable to burn.
Instead: Alkenes are feedstock for ethanol and polymers; alkanes are the fuels.
Mistake: Drawing C₂H₄ as CH₃–CH₃ (or counting C₂H₄ as an alkane).
That formula belongs to ethane.
Instead: Ethene is H₂C=CH₂ — the double bond is the whole point.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
Recall crude oil nature
Crude oil is best described as
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Practise
Topical questions
Exam-style practice in three tiers, from recall to data questions.
- 4Foundation
- 5Standard
- 3Challenging
12 questions worth 49 marks, each with a model answer, mark scheme and examiner tips.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a school paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Chapter 9 Practice Paper
30 marks · 40 min · 4 pages
5 multiple-choice questions and structured questions with answer lines. Chapter 9 (fractions, alkanes, alkenes, bromine water, cracking).
Mark scheme
PROChapter 9 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Where this chapter is tested
Each school sets its own scope — check your assessment outline.
Questions
Frequently asked questions
What does Sec 4 Chemistry Chapter 9 cover?
It covers the first half of section 7 of O-Level Pure Chemistry 6092: crude oil and fractional distillation, the alkane family (CₙH₂ₙ₊₂, combustion, substitution), the alkene family (CₙH₂ₙ, addition, bromine water test), and cracking with its equations.
What is the fastest way to tell an alkane from an alkene?
Shake with orange bromine water in the dark: instant decolourisation means an alkene (addition across C=C); no change means an alkane — it only substitutes slowly, and only in sunlight.
Why does cracking always give one alkane and one alkene?
A big alkane splits a C–C bond; one fragment keeps its saturation (the shorter alkane) and the other end is left one hydrogen short per carbon — a double bond opens, forming the alkene. Atom counting confirms every split.
Which fraction is cracked, and why?
Usually naphtha and the heavier fractions (diesel, fuel oil): fractionation over-supplies big molecules while demand concentrates on petrol-sized alkanes and on alkenes for plastics and ethanol.
When is this chapter usually tested in school?
Organic chemistry opens late in Sec 4 and is a prelim/O-Level staple — formula families, bromine water and cracking equations. Scope varies by school, so check your school’s assessment outline.
Up next
Chapter 10: Alcohols, Carboxylic Acids, Esters and Macromolecules