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As hydrocarbon chain length increases, boiling point increases, viscosity increases and flammability decreases. Free GCSE Chemistry notes and 15 FAQs.
How hydrocarbon size affects properties is part of Carbon compounds as fuels and feedstock in GCSE Edexcel Higher Chemistry.
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As hydrocarbon chain length increases, boiling point increases, viscosity increases and flammability decreases. Free GCSE Chemistry notes and 15 FAQs.
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Use this lesson to stay inside Carbon compounds as fuels and feedstock while connecting the detail back to the wider Topic 8: Fuels and Earth science topic.
Read the core explanations from How hydrocarbon size affects properties before testing yourself from memory.
Picture a candle flame versus a car engine. Both burn hydrocarbons, but they use completely different types. The candle burns small, light molecules that ignite easily. The car engine uses heavier molecules that flow like treacle but pack more energy.
This isn't random. As hydrocarbon molecules get bigger, three key properties change in predictable ways: boiling point, viscosity, and flammability. These changes determine whether a hydrocarbon ends up in your lighter fuel or your car's engine oil.
Think of pouring water versus golden syrup. Water has low viscosity and pours instantly. Syrup has high viscosity and flows slowly. Longer hydrocarbon chains behave more like syrup.
As hydrocarbon chains get longer, the molecules have stronger intermolecular forces between them. These forces are like invisible glue holding the molecules together. More carbons mean more contact points, so stronger forces.
Boiling Point Trend: Longer chains need more energy to break apart and become gas. Methane ($CH_4$) boils at -162°C. Butane ($C_4H_{10}$) boils at -1°C. Octane ($C_8H_{18}$) boils at 126°C.
Viscosity Trend: Longer molecules get tangled up with each other more easily. Short chains like petrol flow freely. Long chains like engine oil are thick and sticky.
Flammability Trend: This one's backwards. Smaller molecules are more flammable because they vaporise easily and mix with oxygen. Larger molecules are harder to ignite.
Worked example: Predicting Properties from Molecular Size
Compare the properties of methane ($CH_4$) and decane ($C_{10}H_{22}$). Predict which has higher boiling point, viscosity, and flammability.
Answer: Decane: higher boiling point, higher viscosity. Methane: more flammable.
These property differences explain why we use different hydrocarbons for different jobs. You wouldn't put engine oil in a lighter, and you wouldn't use lighter fuel to lubricate an engine.
Petrol (C5-C8): Low boiling point means it vaporises easily in the engine. High flammability means it ignites quickly when sparked. Low viscosity means it flows through fuel lines easily.
Diesel (C10-C20): Higher boiling point and lower flammability make it safer to store. Still flows well enough to be pumped, but doesn't ignite as easily as petrol.
Heavy fuel oil (C20+): Very high viscosity means it needs heating to flow. Low flammability makes it safe for large ships, but it won't work in a car engine.
Exam questions often ask you to predict properties or explain why certain hydrocarbons are used as fuels. The key is remembering which properties increase and which decrease with molecular size.
Exam precision note: In How hydrocarbon size affects properties - Properties of hydrocarbons - Carbon compounds as fuels and feedstock, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
The boiling-point trend in hydrocarbons is not found by calculation but by comparing molecular size and intermolecular forces. As chain length increases, there are more electrons and a larger surface area, so intermolecular forces between molecules become stronger. That means more energy is needed to separate the molecules during boiling. The same increase in chain length also increases viscosity because longer molecules can tangle more easily, while flammability decreases because larger molecules are harder to vaporise and ignite.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Viscosity is how thick or runny a liquid is. High viscosity means it flows slowly like honey, and low viscosity means it flows quickly like water. Longer hydrocarbon chains have higher viscosity because the molecules tangle together more.
Heavy fuel oil (C20+) is used for large ships. It has very high viscosity, so it needs heating to flow, and low flammability makes it safe, but it will not work in a car engine.
Flammability decreases with chain length because larger molecules are harder to vaporise and ignite. Longer chains have stronger intermolecular forces, so they do not turn into gas and mix with oxygen as easily.
Diesel (C10-C20) has a higher boiling point and lower flammability than petrol, making it safer to store. It still flows well enough to be pumped but does not ignite as easily as petrol.
Viscosity increases with chain length because longer molecules get tangled up with each other more easily and have stronger intermolecular forces. This is why engine oil is thick while petrol flows freely.
As hydrocarbon chain length increases, boiling point increases, viscosity increases and flammability decreases. Longer chains have stronger intermolecular forces, tangle together more easily and are harder to vaporise and ignite.
Boiling point increases with chain length because longer molecules have more electrons and a larger surface area, so intermolecular forces between them are stronger and more energy is needed to separate them. Methane boils at -162°C, butane at -1°C and octane at 126°C.
Shorter hydrocarbons are more flammable because their small molecules vaporise easily and mix with oxygen, so they ignite at lower temperatures. Larger molecules are harder to vaporise and ignite.
Petrol (C5-C8) is used in cars because its low boiling point means it vaporises easily in the engine, its high flammability means it ignites quickly when sparked, and its low viscosity means it flows through fuel lines easily.
The three major properties affected by hydrocarbon molecule size are boiling point, flammability and viscosity. As molecules get bigger, boiling point and viscosity increase while flammability decreases.
As the molecular size of an alkane increases, its flammability decreases. Smaller molecules are more flammable because they vaporise easily and mix with oxygen, while larger molecules are harder to ignite.
Shorter hydrocarbons suit car engines and stoves because they have low boiling points, so they vaporise easily, and high flammability, so they ignite quickly. Their low viscosity also means they flow through fuel lines easily.
Larger hydrocarbons are more viscous than smaller ones. Longer molecules get tangled up with each other more easily, so long chains like engine oil are thick and sticky while short chains like petrol flow freely.
Methane (CH₄) boils at -162°C, butane (C₄H₁₀) at -1°C and octane (C₈H₁₈) at 126°C. Longer chains need more energy to break apart and become gas.
Decane (C₁₀H₂₂), with 10 carbons, has a higher boiling point and higher viscosity than methane (CH₄), which has 1 carbon. Methane is more flammable because its small molecules vaporise and ignite more easily.