Petroleum, Alkanes and Alkenes

This guide covers Topic 11, Organic chemistry — subtopics 11.3 Fuels, 11.4 Alkanes and 11.5 Alkenes — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. It assumes the vocabulary from Organic Chemistry: Formulae and Naming — functional group, saturated/unsaturated, general formula.

Where this fits in 0620/5070

These three subtopics tell one connected story: petroleum is separated into useful hydrocarbon fractions, the smaller saturated ones (alkanes) are relatively unreactive, and cracking the larger ones deliberately produces unsaturated alkenes, which are far more reactive and far more useful as starting materials — including for the Polymers subtopic that follows.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620

Core

• Naming the fossil fuels coal, natural gas and petroleum, and methane as the main constituent of natural gas (11.3)
• Stating that hydrocarbons contain hydrogen and carbon only, and that petroleum is a mixture of hydrocarbons (11.3)
• Describing the separation of petroleum into fractions by fractional distillation, and how fraction properties change up the column (11.3)
• Naming the uses of each named fraction (11.3)
• Stating that alkane bonding is single covalent, and alkanes are saturated (11.4)
• Describing alkanes as generally unreactive except in combustion and chlorine substitution (11.4)
• Stating that alkenes contain a C=C double bond and are unsaturated (11.5)
• Describing the manufacture of alkenes and hydrogen by cracking larger alkanes, and why cracking is done (11.5)
• Describing the bromine water test to distinguish saturated from unsaturated hydrocarbons (11.5)

11.3 has no Extended-only content at all — every 11.3 outcome above is Core, for every 0620 candidate.

Supplement / Extended

• Describing substitution: one atom/group replaced by another; describing the chlorine-substitution reaction of alkanes as photochemical (UV-initiated), limited to monosubstitution (11.4)
• Describing addition reactions of alkenes (only one product forms) with bromine/aqueous bromine, hydrogen (nickel catalyst) and steam (acid catalyst), and drawing the products (11.5)

CAMBRIDGE O LEVEL CHEMISTRY 5070

5070 has no Core/Extended split — every outcome above, Core and Supplement alike, is required for every O Level candidate.

Petroleum and fractional distillation

Petroleum is a mixture of hydrocarbons — compounds containing hydrogen and carbon only — formed over millions of years from ancient organic matter, alongside coal and natural gas (mainly methane) as the other named fossil fuels.

Fractional distillation separates petroleum into fractions of similar chain length, exploiting the fact that boiling point rises with chain length: crude oil is heated, and the vapours rise through a fractionating column, condensing back to liquid at different heights as they cool — the shortest, most volatile molecules travel highest before condensing; the longest, least volatile molecules condense near the bottom.

Property trend up the column, from bottom to top:

decreasing chain length → higher volatility → lower boiling point → lower viscosity

Named fractions and their uses (bottom to top of the column is the order they’re usually listed, longest chains first):

Fraction	Use
Bitumen	Making roads
Lubricating oil	Lubricants, waxes, polishes
Fuel oil	Fuel for ships, home heating
Diesel oil/gas oil	Fuel for diesel engines
Kerosene/paraffin	Jet fuel
Naphtha	Chemical feedstock
Gasoline/petrol	Fuel for cars
Refinery gas	Heating and cooking gas

Alkanes: structure and reactivity

Alkanes are saturated hydrocarbons — every carbon–carbon bond is a single covalent bond, following the general formula CₙH₂ₙ₊₂ (see Formulae and Naming). This saturation is exactly why alkanes are generally unreactive: there’s no C=C double bond available to react across, unlike alkenes.

The two reactions alkanes do undergo:

• Combustion — burning in oxygen, the basis of their use as fuels.
• Substitution with chlorine — one hydrogen atom is replaced by one chlorine atom. This is a photochemical reaction: ultraviolet light provides the activation energy needed to start it. At this level, the reaction is limited to monosubstitution (replacing exactly one hydrogen).

CH4 + Cl2  --UV-->  CH3Cl + HCl

Cracking: alkenes from alkanes

Cracking breaks larger, less useful alkane molecules into smaller alkanes and alkenes, using a high temperature and a catalyst. Cracking can also produce hydrogen as one of the smaller products, when the alkane chain is broken in a way that leaves a hydrogen atom rather than a second hydrocarbon fragment — for example, decane can be cracked into octane and ethene, or into other combinations that release H2. It’s done because the fractional-distillation supply of short-chain hydrocarbons (especially the highly demanded petrol fraction) doesn’t naturally match demand — cracking converts excess long-chain fractions into the shorter molecules, reactive alkenes, and hydrogen, that are actually wanted.

Alkenes: structure and reactivity

Alkenes contain a C=C double bond, making them unsaturated (general formula CₙH₂ₙ). That double bond is far more reactive than a single bond, which is why alkenes undergo addition reactions — reactions where only one product forms, as atoms add directly across the double bond.

Addition reactions to know:

with bromine/aqueous bromine:      C=C + Br2 → C-C(Br)(Br)     (decolourises orange bromine water)
with hydrogen (nickel catalyst):   C=C + H2  → C-C(H)(H)        (hydrogenation)
with steam (acid catalyst):        C=C + H2O → C-C(H)(OH)       (produces an alcohol)

Testing for saturation: the bromine water test

Shaking a hydrocarbon with aqueous bromine (bromine water) distinguishes saturated from unsaturated compounds directly by colour change:

alkene (unsaturated):  orange bromine water → colourless   (addition reaction occurs)
alkane (saturated):    orange bromine water stays orange   (no reaction)

This is the standard practical test examiners expect you to describe, including both the observation and the reasoning behind it.

Common mistakes

• Describing alkane combustion as the only “chemical property” worth mentioning. Chlorine substitution is equally examinable, and is frequently the one students forget.
• Calling the alkane–chlorine reaction “addition.” It’s substitution — one atom replaces another; nothing is added across a bond, because alkanes have no double bond to add across.
• Forgetting cracking needs a catalyst and heat, not just heat. Both conditions are required and both are examinable.
• Mixing up which test detects unsaturation. Bromine water decolourises in the presence of a C=C double bond (alkenes); it stays orange with alkanes. Getting the colour change backwards is a very common error.
• Writing the hydration of ethene as if it were combustion. Addition of steam across the double bond (acid catalyst) produces ethanol — a completely different reaction from burning.

Quick revision checklist

• Petroleum as a mixture of hydrocarbons, and how fractional distillation separates it
• Named fractions, in order, each with its main use
• Alkanes: saturated, generally unreactive, and their two named reactions
• (0620 Extended, 5070 required) the chlorine-substitution mechanism as photochemical, limited to monosubstitution
• Why cracking is carried out, and the conditions it needs
• Alkenes: unsaturated, and why that makes them more reactive than alkanes
• (0620 Extended, 5070 required) the three named addition reactions and their products
• The bromine water test, both the method and which way the colour changes

Related resources

• Organic Chemistry: Formulae and Naming — the vocabulary this subtopic assumes
• Alcohols and Carboxylic Acids — ethanol, made industrially from ethene by the steam-addition reaction above
• Polymers — addition polymerisation, built directly from alkene monomers
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.