Properties of hydrocarbons — AQA GCSE Combined Science
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Properties of hydrocarbons explained
This statement explains that the physical and chemical behaviour of hydrocarbons is linked to molecule size.
Read the full explanation
As the number of carbon atoms in a hydrocarbon chain increases, the molecules become larger and heavier. This changes three key properties: boiling point increases, viscosity increases and flammability decreases. These trends arise because larger molecules have stronger intermolecular forces between them, so more energy is needed to separate them and they flow less easily. The properties then determine how a hydrocarbon is used. For example, short-chain hydrocarbons such as methane and propane have low boiling points and are highly flammable, making them useful as gases for heating and cooking. Longer-chain hydrocarbons are more viscous and less flammable, so they are used as lubricants or as fuels where a slower, steadier burn is needed.
Students should be able to recall how boiling point, viscosity and flammability change with increasing molecular size.
As hydrocarbon molecules get larger, the chains become longer and the molecules have a greater surface area of contact. This strengthens the intermolecular forces between molecules, so more energy is needed to separate them. Therefore boiling point increases with increasing molecular size. Larger molecules also flow less easily because the stronger intermolecular forces resist sliding, so viscosity increases. Flammability decreases because larger molecules need more energy and oxygen to vaporise and burn, so they ignite less readily. For example, methane (CH₄) is a gas that ignites easily, whereas larger alkanes such as decane (C₁₀H₂₂) are viscous liquids that are harder to ignite.
The combustion of hydrocarbon fuels releases energy. During combustion, the carbon and hydrogen in the fuels are oxidised. The complete combustion of a hydrocarbon produces carbon dioxide and water.
Hydrocarbon fuels store chemical energy in their bonds. When they burn, they react with oxygen from the air and release energy, usually as heat and light. In the reaction, each carbon atom is oxidised to form carbon dioxide and each hydrogen atom is oxidised to form water. Complete combustion happens when there is plenty of oxygen, so the only products are carbon dioxide and water. For example, burning methane: CH₄ + 2O₂ → CO₂ + 2H₂O. The carbon in CH₄ is oxidised to CO₂ and the hydrogen is oxidised to H₂O. If oxygen is limited, combustion is incomplete and carbon monoxide or carbon particles may form instead.
Students should be able to write balanced equations for the complete combustion of hydrocarbons with a given formula.
Complete combustion of a hydrocarbon occurs when it burns in a plentiful supply of oxygen, producing carbon dioxide and water only. To write a balanced equation, first write the correct molecular formula of the hydrocarbon, then place O₂ on the left and CO₂ and H₂O on the right. Balance carbon atoms first, then hydrogen atoms, and finally oxygen atoms, using whole-number coefficients. For example, methane CH₄ burns as CH₄ + 2O₂ → CO₂ + 2H₂O. For ethane C₂H₆, balance to 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Check that every element has the same number of atoms on both sides. State symbols may be added if required, but the essential skill is correct formulae and whole-number balancing.
boiling points
Boiling point is the temperature at which a liquid hydrocarbon changes to gas, with all molecules gaining enough energy to overcome intermolecular forces. In a homologous series, chain length controls boiling point: larger molecules have greater surface contact and stronger intermolecular forces, so more energy is needed to separate them. For example, methane boils near −162 °C, propane near −42 °C and octane near 126 °C. Students should describe the trend, explain it using molecular size and intermolecular forces, and use it to predict states at a stated temperature. Fractional distillation depends on this trend, because shorter chains rise higher in the column and condense at lower temperatures.
viscosity
Viscosity describes how easily a liquid hydrocarbon flows. A viscous liquid is thick and flows slowly because its molecules are long and become tangled, so they resist sliding over one another. In a homologous series, viscosity increases as chain length increases: methane and propane are gases, petrol contains shorter chains and flows easily, while diesel and lubricating oil contain longer chains and are more viscous. Students should describe the trend, explain it using molecular size and intermolecular forces, and relate it to the uses of fractions. For example, petrol is used as fuel because it flows easily, whereas lubricating oil is used where a thick, slow-flowing liquid is needed.
flammability.
Flammability is a chemical property describing how readily a hydrocarbon fuel burns in oxygen. In GCSE Combined Science, you relate flammability to molecular size and intermolecular forces. Short-chain hydrocarbons such as methane, CH₄, and propane, C₃H₈, are highly flammable gases because they have weak intermolecular forces, so they vaporise easily and mix rapidly with oxygen. Longer-chain hydrocarbons such as decane, C₁₀H₂₂, are less flammable liquids because stronger intermolecular forces make them harder to vaporise. Flammability is assessed by comparing how easily fuels ignite and burn, and by linking this to their uses as fuels. Complete combustion produces carbon dioxide and water; incomplete combustion produces carbon monoxide and soot.
Your focus
- Describe the trends in boiling point, viscosity and flammability as hydrocarbon molecule size increases.
- Explain how intermolecular forces account for these trends.
- Relate the properties of hydrocarbons to their uses as fuels or lubricants.
Show all 21 objectives
- State how boiling point changes with increasing molecular size.
- State how viscosity changes with increasing molecular size.
- State how flammability changes with increasing molecular size.
- Describe that combustion of hydrocarbon fuels releases energy.
- Explain that carbon and hydrogen in fuels are oxidised during combustion.
- State the products of the complete combustion of a hydrocarbon as carbon dioxide and water.
- Write a balanced symbol equation for the complete combustion of a given hydrocarbon.
- Apply the law of conservation of mass to check that all atoms are conserved in a combustion equation.
- Deduce the correct coefficients for O₂, CO₂ and H₂O from the molecular formula of a hydrocarbon.
- Describe the trend in boiling points of hydrocarbons as chain length increases.
- Explain the trend in terms of intermolecular forces and energy needed to overcome them.
- Use boiling point data to predict the physical state of a hydrocarbon at a given temperature.
- Describe how viscosity changes as the chain length of a hydrocarbon increases.
- Explain viscosity in terms of molecular size, tangling and intermolecular forces.
- Relate the viscosity of a fraction to its use as a fuel or lubricant.
- Define flammability as a chemical property of hydrocarbons and distinguish it from physical properties.
- Explain how chain length and intermolecular forces affect the flammability of hydrocarbons.
- Compare complete and incomplete combustion in terms of the products formed and the observable flame.
Properties of hydrocarbons exam tips
Marking Points
- State that as hydrocarbon molecules get larger, boiling point increases.
- Explain that larger molecules have stronger intermolecular forces, so more energy is needed to overcome them.
- Describe that viscosity increases as molecule size increases, meaning larger hydrocarbons flow less easily.
- Explain that flammability decreases as molecule size increases, so larger hydrocarbons are harder to ignite.
- Link these properties to uses: small molecules as fuels for burning, larger molecules as lubricants or for slower-burning fuels.
- Use the trends to predict the behaviour of an unfamiliar hydrocarbon when its size is given.
- Boiling point increases as molecular size increases because intermolecular forces become stronger with longer chains.
- Viscosity increases as molecular size increases because larger molecules flow less easily.
- Flammability decreases as molecular size increases because larger molecules are harder to ignite and burn.
- The trend applies across a homologous series such as the alkanes, where each member differs by a CH₂ unit.
- Intermolecular forces, not covalent bonds, are overcome during boiling; covalent bonds within molecules remain intact.
- A named example can support the trend, such as methane being a flammable gas and decane being a viscous, less flammable liquid.
- Combustion of hydrocarbon fuels is an exothermic reaction that releases energy.
- Carbon in the fuel is oxidised to carbon dioxide during complete combustion.
- Hydrogen in the fuel is oxidised to water during complete combustion.
- Complete combustion requires a good supply of oxygen and produces carbon dioxide and water only.
- A balanced symbol equation can represent the reaction, such as CH₄ + 2O₂ → CO₂ + 2H₂O.
- Oxidation in this context means reaction with oxygen, so the carbon and hydrogen atoms gain oxygen.
- Correctly identify the products of complete combustion as carbon dioxide and water.
- Write the correct molecular formula for the given hydrocarbon, including any double bonds implied by the formula.
- Balance carbon atoms by adjusting the coefficient of CO₂ to match the number of carbon atoms in the hydrocarbon.
- Balance hydrogen atoms by adjusting the coefficient of H₂O to match half the number of hydrogen atoms in the hydrocarbon.
- Balance oxygen atoms last by adjusting the coefficient of O₂, using a whole number or an even-numbered coefficient where necessary.
- Boiling point increases as the number of carbon atoms in the hydrocarbon chain increases.
- Larger hydrocarbon molecules have greater intermolecular forces because they have more surface contact between molecules.
- More energy is required to overcome these intermolecular forces, so the boiling point is higher.
- Smaller hydrocarbons such as methane and propane are gases at room temperature, while larger ones such as octane are liquids.
- The trend applies within a homologous series and can be used to predict the physical state at a given temperature.
- Fractional distillation separates hydrocarbons because fractions with lower boiling points condense higher up the column.
- Viscosity is a measure of how easily a liquid flows; higher viscosity means slower flow.
- As the number of carbon atoms in the hydrocarbon chain increases, viscosity increases.
- Longer hydrocarbon molecules become tangled and have stronger intermolecular forces, so they resist flowing past one another.
- Shorter hydrocarbons are less viscous and flow more easily, which is why petrol is more runny than diesel or lubricating oil.
- Viscosity is a physical property that helps determine the use of a fraction, such as fuel or lubricant.
- The trend can be observed by comparing how quickly equal volumes of different fractions flow through the same tube or opening.
- Flammability is a chemical property: it describes how easily a substance burns in oxygen, not how it looks or melts.
- Hydrocarbons are flammable because they react exothermically with oxygen in combustion, releasing energy as heat and light.
- Short-chain hydrocarbons are more flammable than long-chain hydrocarbons because they have weaker intermolecular forces and lower boiling points, so they vaporise and mix with oxygen more readily.
- Complete combustion of a hydrocarbon produces carbon dioxide and water; incomplete combustion produces carbon monoxide and/or carbon (soot).
- Flammability influences the choice of fuel: gases such as methane and propane are used where rapid ignition and a clean flame are needed, while longer-chain liquids are used where a less volatile fuel is safer or more convenient.
Examiner Tips
- 💡When explaining a trend, always refer to the size of the molecule and the strength of intermolecular forces.
- 💡Use comparative language such as 'higher', 'lower', 'more viscous' and 'less flammable' to make your answers precise.
- 💡If asked to suggest a use, match the property to the job: highly flammable for easy burning, viscous for lubrication.
- 💡Practise drawing or interpreting a graph of boiling point against number of carbon atoms to reinforce the trend.
- 💡Use the phrase 'increasing molecular size' consistently and state the direction of each trend clearly.
- 💡Link each property to intermolecular forces rather than to covalent bond strength.
- 💡Give a specific example from the alkanes to show the trend, such as methane compared with a larger alkane.
- 💡Name both products of complete combustion: carbon dioxide and water.
- 💡Use a balanced equation to show the reactants and products clearly.
- 💡State that combustion is exothermic and that energy is released to the surroundings.
- 💡Write the unbalanced word or formula equation first, then balance systematically rather than guessing coefficients.
- 💡Show your balancing steps clearly so that a correct final equation is not lost through an arithmetic slip.
- 💡When describing the trend, quote at least two named hydrocarbons and their approximate boiling points to support your answer.
- 💡Use the phrase intermolecular forces rather than bonds when explaining why boiling points increase.
- 💡If asked to predict a state, compare the boiling point with the stated temperature and state whether the substance is solid, liquid or gas.
- 💡Link boiling point to fractional distillation by explaining that shorter chains condense at lower temperatures higher up the column.
- 💡Define viscosity clearly before describing the trend, so your answer has a secure starting point.
- 💡Compare named fractions such as petrol, diesel and lubricating oil to make the trend concrete.
- 💡Explain viscosity using molecular size, tangling and intermolecular forces rather than using the word thick alone.
- 💡When linking viscosity to use, state the required property, such as easy flow for a fuel or slow flow for a lubricant.
- 💡When asked to compare flammability, link it to chain length, intermolecular forces and boiling point rather than just saying 'smaller is more flammable'.
- 💡Use the correct products for complete and incomplete combustion; if a question mentions a yellow smoky flame or limited air, think carbon monoxide and soot.
- 💡Apply flammability to real fuels: explain why methane is used in domestic gas and why longer-chain hydrocarbons are used as diesel or lubricants.
Common Mistakes
- Error: saying that boiling point decreases as molecule size increases. Correction: boiling point increases because larger molecules have stronger intermolecular forces.
- Error: confusing viscosity with density. Correction: viscosity is resistance to flow, not mass per unit volume.
- Error: thinking all hydrocarbons are equally flammable. Correction: flammability decreases as molecule size increases.
- Error: assuming larger hydrocarbons are always better fuels. Correction: larger hydrocarbons are less flammable and may be used as lubricants instead.
- Saying boiling point decreases with molecular size: correct this by linking longer chains to stronger intermolecular forces and more energy needed to boil.
- Confusing viscosity with flammability: correct this by stating that viscosity is resistance to flow, while flammability is how easily a substance catches fire.
- Claiming covalent bonds break during boiling: correct this by explaining that only intermolecular forces are overcome when a simple molecular substance boils.
- Writing water as the only product: correct this by including carbon dioxide as well.
- Saying energy is destroyed or used up: correct this by stating that energy is transferred to the surroundings during combustion.
- Confusing complete and incomplete combustion: correct this by stating that complete combustion produces carbon dioxide and water, while incomplete combustion can produce carbon monoxide or carbon.
- Writing water as OH or hydrogen peroxide instead of H₂O; correct by using H₂O for the combustion product.
- Balancing oxygen before carbon and hydrogen, which often leads to fractions; correct by balancing C, then H, then O.
- Forgetting to multiply the oxygen count in CO₂ and H₂O when choosing the O₂ coefficient; correct by counting all oxygen atoms on the product side before setting the O₂ coefficient.
- Error: stating that covalent bonds inside hydrocarbon molecules break during boiling. Correction: boiling overcomes intermolecular forces between molecules; covalent bonds remain intact.
- Error: saying larger molecules have weaker forces because they are heavier. Correction: larger molecules have stronger intermolecular forces because of greater surface contact, despite being heavier.
- Error: confusing boiling point with melting point or with the temperature at which a fraction condenses. Correction: boiling point is the temperature at which a liquid changes to gas at a stated pressure.
- Error: saying viscosity increases because molecules are heavier. Correction: viscosity increases mainly because longer molecules become tangled and have stronger intermolecular forces, not simply because they are heavier.
- Error: confusing viscosity with boiling point. Correction: viscosity describes resistance to flow, while boiling point describes the temperature of a change of state.
- Error: stating that viscous liquids are denser. Correction: viscosity and density are different properties; a liquid can be viscous without being unusually dense.
- Error: stating that flammability is a physical property. Correction: flammability is a chemical property because burning produces new substances.
- Error: claiming that long-chain hydrocarbons are more flammable because they contain more carbon. Correction: longer chains have stronger intermolecular forces, so they are less volatile and generally less flammable.
- Error: writing that combustion always produces carbon dioxide and water. Correction: incomplete combustion can also produce carbon monoxide and soot, especially when oxygen supply is limited.