Organic chemistry: GCSE Combined Science
Crude oil, alkanes, fractional distillation, cracking, alkenes and combustion
The short version
Alkanes are saturated C_nH_{2n+2}. Cracking makes smaller useful molecules including alkenes. Alkenes decolourise bromine water because they are unsaturated.
Key idea
Crude oil has no fixed formula because it is a mixture
Grade 2–3: Oil comes from tiny sea plants and animals from long ago. It is a mix, not one pure chemical.
Grade 5–6: I can define hydrocarbon and finite, and I know we separate oil because chain lengths differ.
Grade 9: Feedstock means a chemical starting material, not just a fuel. Cracking later turns low-demand long molecules into high-demand short ones.
Go a bit deeper
Each member differs by CH2. Same general formula, similar chemistry
Grade 2–3: Methane, ethane, propane, butane. Long ones are sticky and do not catch fire easily.
Grade 5–6: I can write C_nH_{2n+2} and the first four formulae.
Grade 9: I never say longer chains have stronger covalent bonds so higher boiling points. Boiling breaks intermolecular forces, not C–C bonds.
Worked example
Worked example
Grade 2–3: Oil comes from tiny sea plants and animals from long ago. It is a mix, not one pure chemical. Grade 5–6: I can define hydrocarbon and finite, and I know we separate oil because chain lengths differ. Grade 9: Feedstock means a chemical starting material, not just a fuel. Cracking later turns low-demand long molecules into high-demand short ones.
Lessons in this topic
- Crude oil
- Hydrocarbons
- Fractional distillation
- Cracking
- Alkenes (Combined depth)
- Combustion
In the app each lesson is video (when we have a real YouTube id) then intro, learn and summary steps.
What the examiner wants
State trends: longer chains have higher boiling points and are less flammable. Cracking conditions: high temperature and a catalyst. Combined does not require alcohols or esters.
Try these questions
Cover the answers and have a go first. Worked solutions are underneath.
A hydrocarbon contains
Crude oil is
The general formula of alkanes is
As alkane chains get longer, boiling point
Common mistakes
- CO2 is not a hydrocarbon — it contains oxygen. Hydrocarbons are H and C only. Crude oil is not a compound.
- Do not say longer chains have stronger covalent bonds so higher boiling points. Boiling breaks intermolecular forces, not C–C bonds.
- Fractional distillation is a physical process. No new substances are made. Cracking (next lesson) is chemical.
- You cannot crack an alkane into two alkanes only and balance hydrogen. You usually need an alkene (fewer hydrogens).
Answers and working
Frequently asked questions
Is GCSE Organic chemistry on Combined Science and Triple?
Organic chemistry is on Combined Science and on Triple Chemistry. Progress is shared in the ReviseEasy app.
Which exam board is this Organic chemistry page for?
The ideas are board-agnostic for AQA, Edexcel, OCR, WJEC and CCEA. Paper lengths and question numbers differ — check your specification.
Is Organic chemistry Foundation or Higher?
The core ideas appear on both tiers. Higher papers add extra application and some HT-only detail, marked in lessons with a Higher badge.
Learn, try, get feedback, fix the mistake
Open the stepped lesson, then the quiz with hints. Required-practical and calculation questions use the same method you will need in the paper.
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