Hydrocarbons and Crude Oil

This lesson covers hydrocarbons, alkanes, and crude oil as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand what hydrocarbons are, be able to name and draw the first four alkanes, and explain why crude oil is such an important resource.

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What Is a Hydrocarbon?

A hydrocarbon is a compound that contains only hydrogen and carbon atoms. There are no other elements present. Hydrocarbons are the main compounds found in crude oil and natural gas.

Key Points

• Hydrocarbons contain only C and H atoms.
• They are covalent compounds — the atoms are held together by shared pairs of electrons.
• The simplest hydrocarbons belong to a family called the alkanes.

Exam Tip: If a compound contains any element other than carbon and hydrogen — for example oxygen or nitrogen — it is not a hydrocarbon.

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Alkanes

Alkanes are a homologous series of hydrocarbons. A homologous series is a family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties.

General Formula

The general formula for alkanes is:

CₙH₂ₙ₊₂

where n is the number of carbon atoms.

The First Four Alkanes

Name	Molecular Formula	Number of C Atoms	Number of H Atoms	State at Room Temperature
Methane	CH₄	1	4	Gas
Ethane	C₂H₆	2	6	Gas
Propane	C₃H₈	3	8	Gas
Butane	C₄H₁₀	4	10	Gas

Displayed Formulae

In a displayed formula you show every bond between every atom:

• Methane (CH₄): A central carbon bonded to four hydrogen atoms.
• Ethane (C₂H₆): Two carbons joined by a single bond, each with three hydrogens.
• Propane (C₃H₈): Three carbons in a chain; the end carbons have three hydrogens each, the middle carbon has two.
• Butane (C₄H₁₀): Four carbons in a chain with the appropriate number of hydrogens on each.

graph LR
    subgraph Methane CH₄
        C1["C"] --- H1["H"]
        C1 --- H2["H"]
        C1 --- H3["H"]
        C1 --- H4["H"]
    end
    subgraph Ethane C₂H₆
        C2a["C"] --- C2b["C"]
        C2a --- H5["H"]
        C2a --- H6["H"]
        C2a --- H7["H"]
        C2b --- H8["H"]
        C2b --- H9["H"]
        C2b --- H10["H"]
    end

Properties of Alkanes

• All alkanes have single covalent bonds only (C–C and C–H).
• They are described as saturated hydrocarbons because every carbon–carbon bond is a single bond — no double bonds.
• As the chain length increases, boiling point increases and the hydrocarbons change from gases → liquids → solids.
• Alkanes are relatively unreactive — they do not react with aqueous reagents — but they burn readily.

Exam Tip: "Saturated" means all carbon–carbon bonds are single bonds. If a hydrocarbon has a C=C double bond it is unsaturated (an alkene, not an alkane).

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Crude Oil

Crude oil is a fossil fuel formed over millions of years from the remains of ancient marine organisms (plankton and algae) that were buried under layers of sediment. Heat and pressure converted the remains into a complex mixture of hydrocarbons.

Crude Oil Is a Mixture

Crude oil is a mixture of many different hydrocarbons — mostly alkanes — with different chain lengths. Because it is a mixture, the components are not chemically bonded to each other and can be separated by physical methods.

Property	Short-Chain Hydrocarbons	Long-Chain Hydrocarbons
Boiling point	Low	High
Viscosity	Runny (low)	Thick / sticky (high)
Flammability	Highly flammable	Less flammable
Colour	Light / pale	Dark
Volatility	High (evaporate easily)	Low

Why Is Crude Oil Important?

Crude oil is one of the most important natural resources because:

• It provides fuels (petrol, diesel, kerosene, fuel oil, LPG).
• It is the feedstock for the petrochemical industry — producing plastics, medicines, dyes, detergents and many other products.
• It is a finite (non-renewable) resource — it took millions of years to form and is being used faster than it can be replaced.

Trends in Properties with Chain Length

graph TD
    A["Increasing chain length"] --> B["Boiling point INCREASES"]
    A --> C["Viscosity INCREASES"]
    A --> D["Flammability DECREASES"]
    A --> E["Volatility DECREASES"]
    A --> F["Colour becomes DARKER"]

    style A fill:#2c3e50,color:#fff
    style B fill:#e74c3c,color:#fff
    style C fill:#e67e22,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#2980b9,color:#fff
    style F fill:#8e44ad,color:#fff

Exam Tip: You must be able to explain the trend in boiling points. Longer chains have stronger intermolecular forces (more points of contact between molecules), so more energy is needed to separate them, giving a higher boiling point.

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Finite Resource and Sustainability

• Crude oil is a non-renewable resource — once used, it cannot be replaced within a human lifetime.
• Burning fossil fuels releases CO₂ (a greenhouse gas) contributing to climate change.
• We depend on crude oil for transport, heating, electricity and manufacturing.
• Scientists are developing alternative fuels (hydrogen, biofuels, electric vehicles) to reduce our dependence on crude oil.

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Summary

• A hydrocarbon contains only carbon and hydrogen atoms.
• Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂.
• The first four alkanes are methane, ethane, propane and butane.
• Crude oil is a finite mixture of hydrocarbons formed from ancient marine organisms.
• As chain length increases: boiling point and viscosity increase; flammability and volatility decrease.
• Crude oil is vital for fuels and as a feedstock for the petrochemical industry.

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Worked Examples

Worked Example 1 — Writing an alkane molecular formula

Question: Write the molecular formula of the alkane with 7 carbon atoms (heptane).

Step 1: Recall the general formula for the homologous series of alkanes: CₙH₂ₙ₊₂.

Step 2: Substitute n = 7: H atoms = (2 × 7) + 2 = 16.

Step 3: Write the formula: C₇H₁₆.

Common Mistake: Learners sometimes write C₇H₁₄ by forgetting the "+2". That would be an alkene (unsaturated), not an alkane (saturated).

Worked Example 2 — Identifying the odd one out

Question: Which of these is NOT a hydrocarbon? CH₄, C₂H₆, CH₃OH, C₄H₁₀.

Answer: CH₃OH (methanol). It contains oxygen as well as carbon and hydrogen, so by definition it is not a hydrocarbon. Hydrocarbons contain only carbon and hydrogen atoms.

Worked Example 3 — Predicting properties from chain length

Question: Compare the boiling point and viscosity of pentane (C₅H₁₂) with that of decane (C₁₀H₂₂).

Answer: Decane has the longer chain, so it has stronger intermolecular forces. More energy is needed to separate decane molecules, giving decane a higher boiling point. Longer molecules also tangle more, so decane has a higher viscosity (flows less easily) than pentane.

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Comparison Tables

Table 1 — Properties of the first four alkanes

Alkane	Formula	State at 20 °C	Boiling point (°C)	Typical use
Methane	CH₄	Gas	−162	Natural gas (heating, cooking)
Ethane	C₂H₆	Gas	−89	Petrochemical feedstock
Propane	C₃H₈	Gas	−42	LPG (camping stoves)
Butane	C₄H₁₀	Gas	−1	Lighters, portable heaters

Table 2 — How properties change down the homologous series

Property	Short chains	Long chains
Boiling point	Low	High
Viscosity	Runny (low)	Thick (high)
Flammability	Very flammable	Harder to ignite
Volatility	High	Low
Colour (of fraction)	Pale/colourless	Dark/brown

Exam Tip: Examiners often ask you to link the trend to intermolecular forces. Longer chains = stronger London dispersion forces = more energy needed to separate molecules.

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Mermaid Diagram — Crude Oil Pathway

flowchart TD
  A[Dead marine organisms] --> B[Buried under sediment]
  B --> C["High pressure and temperature<br/>millions of years"]
  C --> D[Crude oil + natural gas]
  D --> E[Extracted by drilling]
  E --> F[Fractional distillation]
  F --> G[Fuels: petrol, diesel, kerosene]
  F --> H[Feedstock: alkenes, polymers, solvents]

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Common Mistakes to Avoid

• Saying crude oil is a compound. It is a mixture — the hydrocarbons are not chemically bonded to each other.
• Writing hydrocarbons as CnH2n. That is the alkene general formula. Alkanes use CₙH₂ₙ₊₂.
• Forgetting that crude oil is finite. It takes millions of years to form, so it is a non-renewable resource.
• Saying short chains "evaporate faster because they are lighter". The correct reason is weaker intermolecular forces, not mass alone.

Answering at different grade levels

Examiners reward precise terminology. Compare these three responses to the question "Why does hexane have a higher boiling point than methane?"

• Grade 3–4 response: "Hexane is bigger than methane, so it takes more heat to boil." This shows basic understanding but lacks key terms.
• Grade 5–6 response: "Hexane has a longer carbon chain than methane. Longer chains have stronger forces between molecules, so more energy is needed to separate them, giving a higher boiling point." This uses the idea of intermolecular forces.
• Grade 7–9 response: "Both hexane and methane are members of the alkane homologous series (general formula CₙH₂ₙ₊₂) and are saturated hydrocarbons. Hexane has a longer chain, giving larger London dispersion forces between molecules. More energy is required to overcome these intermolecular forces, so hexane has a higher boiling point. The functional group (C–H and C–C single bonds) is the same in both, so the trend is driven by chain length, not bonding type." This uses terms such as homologous series, functional group, saturated and links intermolecular forces to the property.

When writing top-band answers, always name the homologous series, identify whether molecules are saturated or unsaturated, and link chain length to physical properties via intermolecular forces.

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Extra Practice — Exam-Style Questions

Q1. Name the first four members of the alkane homologous series and give their molecular formulae.

Answer: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀. Each differs by CH₂ from the next, which is a defining feature of a homologous series.

Q2. Explain why crude oil is described as a finite resource and a non-renewable resource.

Answer: Crude oil takes hundreds of millions of years to form from buried marine organisms under heat and pressure. We are using it far faster than it can be replaced, so its supply is finite and it cannot be renewed within human timescales.

Q3. A hydrocarbon has the molecular formula C₈H₁₈. State whether it is an alkane or an alkene, giving a reason.

Answer: Alkane. Applying the alkane general formula CₙH₂ₙ₊₂ for n = 8 gives 18 H, matching C₈H₁₈. It is therefore saturated with no C=C functional group.

Q4. State three physical properties that change as the length of a hydrocarbon chain increases.

Answer: (1) Boiling point increases; (2) Viscosity increases (molecules are thicker/stickier); (3) Flammability decreases (and volatility decreases — longer chains evaporate less easily).

Q5. Crude oil is a mixture, not a compound. What is the difference?

Answer: A compound contains elements chemically bonded in fixed proportions (e.g. water H₂O). A mixture contains different substances not chemically bonded that can be separated by physical means (e.g. fractional distillation of crude oil).

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Key Terms Glossary

• Hydrocarbon — a compound containing only hydrogen and carbon atoms.
• Alkane — a saturated hydrocarbon with general formula CₙH₂ₙ₊₂. All C–C and C–H bonds are single bonds.
• Alkene — an unsaturated hydrocarbon with general formula CₙH₂ₙ, containing at least one C=C functional group.
• Homologous series — a family of compounds with the same general formula, similar chemical properties, and a gradual trend in physical properties.
• Saturated — containing only single bonds between carbon atoms; no C=C.
• Unsaturated — containing at least one double (or triple) bond between carbon atoms.
• Functional group — the atom or group of atoms that gives a compound its characteristic reactions (e.g. C=C in alkenes).
• Crude oil — a finite, non-renewable mixture of hydrocarbons formed from ancient marine organisms.
• Finite resource — a resource that is being used up faster than it is formed.
• Viscosity — the resistance of a liquid to flow.
• Volatility — the tendency of a liquid to evaporate.
• Intermolecular forces — forces between molecules (not inside molecules); for hydrocarbons, these are mainly London dispersion forces.

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Revision Checklist

Before moving on, make sure you can confidently:

• Define hydrocarbon, alkane, homologous series, saturated and unsaturated.
• Write the molecular formulae and displayed formulae of methane, ethane, propane and butane.
• Use the general formula CₙH₂ₙ₊₂ to predict the formula of any alkane.
• Explain why crude oil is a finite, non-renewable mixture and how it was formed from ancient marine organisms over millions of years.
• Describe and explain the trends in boiling point, viscosity, flammability and volatility as chain length increases — linking each trend to intermolecular forces.
• Give uses of crude oil as both a fuel and a feedstock for the petrochemical industry.
• Identify whether a molecule is a hydrocarbon from its formula.

Use this checklist as a quick self-test before exam day; tick each bullet only once you can explain it out loud without notes.

Edexcel alignment: This content is aligned with Edexcel GCSE Combined Science (1SC0) Chemistry Topic 8 Fuels and Earth science / Topic 9 Hydrocarbons — specifically CC15 Fuels (hydrocarbons, alkanes, crude oil as a finite resource), CC13 Groups in the periodic table (links to bonding patterns), and the introduction to CC16 Atmospheric pollution (consequences of combustion). Assessed on Chemistry Paper 2.