✓ Revise Right course
An alkane is a saturated hydrocarbon with only single bonds between carbon atoms and general formula CnH2n+2. Free notes and 10 questions answered.
Understanding alkanes - the main hydrocarbons in crude oil is part of Carbon compounds as fuels and feedstock in GCSE AQA Higher Chemistry.
4 min
Reading time
23
Lessons in this topic
GCSE AQA Higher
Pathway
Reviewed by Revise Right
Content review
An alkane is a saturated hydrocarbon with only single bonds between carbon atoms and general formula CnH2n+2. Free notes and 10 questions answered.
AQA GCSE Chemistry mock draft, 26–31 Oct. Draft coverage: Chemistry Paper 1 (Higher tier). Reviewed for an original-practice sitting; check student availability before registering. Marking and results will follow later, after separate approval. Eligible new accounts receive a five-day Pro trial when signup email verification completes. Previous trials keep their original expiry. After expiry, new papers need Pro unless you have an explicitly confirmed free sitting offer for this event. That offer adds no Pro time or AI learning. Saved attempts and released results remain readable. No card or automatic charge.
Use this lesson to stay inside Carbon compounds as fuels and feedstock while connecting the detail back to the wider Topic 7: Organic chemistry topic.
Read the core explanations from Understanding alkanes - the main hydrocarbons in crude oil before testing yourself from memory.
When you fill up a car with petrol or heat your home with natural gas, you're using alkanes. These simple molecules make up most of crude oil and are the backbone of our energy system. But what exactly are they?
The key word here is 'saturated' - this means every carbon atom forms four single bonds, with no double or triple bonds anywhere. Think of it like a carbon skeleton that's completely filled up with hydrogen atoms.
**Trap**: Many students think 'saturated' means 'dissolved in water'. Actually, it means the carbon chain is saturated with hydrogen - no more can be added without breaking bonds.
Alkanes form what we call a homologous series. Each member has the same general formula but differs by exactly one CH2 group. Let's meet the first four members of this family.
Methane (CH4): The simplest alkane with just one carbon atom. This is natural gas - what heats many homes and powers gas hobs.
Ethane (C2H6): Two carbon atoms joined together. Found in natural gas mixtures and used to make plastics.
Propane (C3H8): Three carbons in a chain. This is the gas in camping stoves and BBQ cylinders.
Butane (C4H10): Four carbons long. Found in lighter fuel and some aerosols.
Notice the pattern? Each time we add one carbon, we also add two hydrogens. This is where the general formula comes from.
Every alkane follows the same pattern. Once you know this general formula, you can work out the molecular formula for any alkane.
Worked example: Using the General Formula
What is the molecular formula for the alkane with 5 carbon atoms?
Answer: C5H12
This formula works because of how carbon bonds. Each carbon forms exactly four bonds - in a chain, the end carbons bond to three hydrogens each, while middle carbons bond to two hydrogens each.
Crude oil formed millions of years ago from dead sea creatures and plants. Under high pressure and temperature, the complex organic molecules broke down into simpler, more stable compounds - mainly alkanes.
Alkanes are chemically stable because all their bonds are single bonds. This stability meant they survived the geological processes that destroyed other types of molecules. That's why when we drill for oil, we find mostly alkanes of different chain lengths.
The different alkanes in crude oil have different boiling points, which is why we can separate them by fractional distillation. Shorter alkanes like methane and ethane boil at low temperatures, while longer chains like those in diesel and fuel oil need much higher temperatures.
As alkanes get longer, their properties change in predictable ways. This is crucial for understanding how crude oil is processed and used.
Boiling Point: Increases as the chain gets longer. Methane boils at -162°C (gas at room temperature), while longer alkanes like those in petrol are liquids.
Viscosity: Longer alkanes are more viscous (thicker). Methane flows like air, petrol flows easily, while heavy fuel oil is quite thick.
Flammability: Shorter alkanes ignite more easily. That's why gas (methane) lights instantly, but diesel needs compression ignition.
Alkane questions often test your ability to use the general formula and recognise patterns. Here's how to nail these questions every time.
Common Exam Mistakes: Don't confuse alkanes (CnH2n+2) with alkenes (CnH2n). Don't say alkanes are 'unsaturated' - they're saturated. Don't forget that crude oil contains mainly alkanes, not all hydrocarbons.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-25.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
The general formula for alkanes is CnH2n+2, where n is the number of carbon atoms. For example, an alkane with 5 carbon atoms has 2(5) + 2 = 12 hydrogen atoms, so its formula is C5H12, which is pentane.
An alkane is a saturated hydrocarbon that contains only single bonds between carbon atoms, with the general formula CnH2n+2. Most of the hydrocarbons in crude oil are alkanes.
Saturated means a hydrocarbon has only single bonds, so every carbon atom forms four single bonds and the carbon chain is saturated with hydrogen. Alkanes are saturated; it does not mean dissolved in water.
Propane (C₃H₈), the alkane with three carbons in a chain, is the gas in camping stoves and BBQ cylinders. It is the third member of the alkane homologous series.
The first four alkanes are methane (CH4), ethane (C2H6), propane (C3H8) and butane (C4H10). Each member of this homologous series has one more CH2 group than the one before.
Butane (C₄H₁₀), an alkane four carbons long, is found in lighter fuel and some aerosols. It is the fourth member of the alkane homologous series, after methane, ethane and propane.
Ethane (C₂H₆), with two carbon atoms joined together, is found in natural gas mixtures and used to make plastics. It is the second alkane in the homologous series.
As alkane chains get longer, the boiling point increases, viscosity increases so they get thicker, and shorter alkanes ignite more easily than longer ones. Methane boils at -162°C and is a gas, while longer alkanes such as those in petrol are liquids.
The molecular formula of pentane, the alkane with 5 carbon atoms, is C₅H₁₂. Using the general formula CₙH₂ₙ₊₂ with n = 5, the number of hydrogen atoms is 2(5) + 2 = 12.
Crude oil is mostly alkanes because alkanes are chemically stable, with only single bonds. Under high pressure and temperature, complex organic molecules from dead sea creatures and plants broke down into simpler compounds, and the stable alkanes survived.