What are alkanes in the context of Organic Chemistry?

Alkanes are a class of hydrocarbons in Organic Chemistry that consist solely of single-bonded carbon and hydrogen atoms. They are saturated hydrocarbons, meaning they have the maximum number of hydrogen atoms attached to each carbon atom. Alkanes follow the general formula CnH2n+2 and are a fundamental topic in the Cambridge IGCSE Chemistry curriculum.

How are alkanes named according to IUPAC nomenclature?

In IUPAC nomenclature, alkanes are named based on the number of carbon atoms in the longest continuous chain. The names typically end in '-ane'. For example, methane (CH4) is the simplest alkane with one carbon atom, ethane (C2H6) has two carbon atoms, and propane (C3H8) has three. This systematic naming helps students in the Cambridge IGCSE Chemistry course to identify and differentiate between various alkanes.

What are the physical properties of alkanes?

Alkanes generally have low melting and boiling points, which increase with the length of the carbon chain. They are non-polar molecules, making them insoluble in water but soluble in organic solvents. These properties are crucial for understanding alkanes in the Cambridge IGCSE Chemistry syllabus, as they influence how alkanes behave in different chemical reactions and environments.

What is the significance of alkanes in the Cambridge IGCSE Chemistry curriculum?

Alkanes are significant in the Cambridge IGCSE Chemistry curriculum as they serve as a foundational concept in Organic Chemistry. Understanding alkanes helps students grasp more complex topics like functional groups, isomerism, and chemical reactions involving hydrocarbons. Alkanes are also important in real-world applications, such as fuels and lubricants, making them relevant for both academic and practical learning.

How do alkanes react with halogens in substitution reactions?

Alkanes undergo substitution reactions with halogens in the presence of ultraviolet light. This process, known as halogenation, involves the replacement of a hydrogen atom in the alkane with a halogen atom, forming a haloalkane. For example, when methane reacts with chlorine, chloromethane is formed. This reaction is a key concept in the Cambridge IGCSE Chemistry curriculum, illustrating the reactivity of alkanes and their ability to form a variety of compounds.

Why is "Saying alkanes undergo addition reactions" a common mistake in Alkanes?

Why it happens: Confusing with alkenes. How to avoid it: Alkanes: SUBSTITUTION (need UV). Alkenes: ADDITION (no UV needed). Source: 0620/42 — recurring

Why is "Forgetting the UV condition for substitution" a common mistake in Alkanes?

Why it happens: Speed. How to avoid it: Always include 'UV' or 'sunlight' over the arrow for halogenation of alkanes.

Why is "Producing only smaller alkanes from cracking" a common mistake in Alkanes?

Why it happens: Forgetting alkenes. How to avoid it: Cracking ALWAYS produces an alkane + at least one alkene (otherwise H atoms don't balance).

Organic ChemistryCambridge IGCSE Chemistry 0620 Extended

Alkanes

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Alkanes — Cambridge IGCSE 0620 Chemistry Extended (2026)

Saturated hydrocarbons with C-C single bonds only. General formula $\text{C}_{n}\text{H}_{2n+2}$ . Combustion and substitution with halogens are the key reactions.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

13.4 — Describe alkanes as saturated hydrocarbons with general formula $\text{C}_{n}\text{H}_{2n+2}$ .
13.4 — Write equations for combustion of alkanes.
13.4 — Describe substitution reactions of alkanes with halogens (in UV).

Structure of alkanes

Carbon atoms in a chain, joined by single bonds, hydrogens fill remaining valences.

Saturated. Each carbon has FOUR single bonds — no double or triple bonds. Each carbon is bonded to as many hydrogens as possible (no room for more) → "saturated".

General formula: $\text{C}_{n}\text{H}_{2n+2}$ .

The first six alkanes.

Name	Formula	Structural
Methane	$\text{CH}_{4}$	$\text{CH}_{4}$
Ethane	$\text{C}_{2}\text{H}_{6}$	$\text{CH}_{3}\text{CH}_{3}$
Propane	$\text{C}_{3}\text{H}_{8}$	$\text{CH}_{3}\text{CH}_{2}\text{CH}_{3}$
Butane	$\text{C}_{4}\text{H}_{10}$	$\text{CH}_{3}\text{CH}_{2}\text{CH}_{2}\text{CH}_{3}$
Pentane	$\text{C}_{5}\text{H}_{12}$	$\text{CH}_{3}(\text{CH}_{2})_{3}\text{CH}_{3}$
Hexane	$\text{C}_{6}\text{H}_{14}$	$\text{CH}_{3}(\text{CH}_{2})_{4}\text{CH}_{3}$

In an alkane every carbon has four single bonds — no spare capacity, so it is saturated.

Properties trends within alkane series.

m.p. and b.p. INCREASE with chain length (stronger intermolecular forces).
Methane to butane: gases. Pentane onwards: liquids (then solids beyond ~ $\text{C}_{18}$ ).
Insoluble in water (non-polar).
Less dense than water.
Burn easily (good fuels).

Saturated: all single bonds.
$\text{C}_{n}\text{H}_{2n+2}$ .
First 6: methane → hexane.
Burn well; otherwise unreactive.
Liquid alkanes float on water.

Combustion of alkanes

Complete: alkane + O₂ → CO₂ + H₂O. Incomplete: CO and/or C.

Complete combustion. Plenty of oxygen. $\text{C}_{n}\text{H}_{2n+2} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}.$

Worked. Methane: $\text{CH}_{4} + 2\text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$ . Worked. Propane: $\text{C}_{3}\text{H}_{8} + 5\text{O}_{2} \to 3\text{CO}_{2} + 4\text{H}_{2}\text{O}$ . Worked. Octane (a gasoline component): $2\text{C}_{8}\text{H}_{18} + 25\text{O}_{2} \to 16\text{CO}_{2} + 18\text{H}_{2}\text{O}$ .

Incomplete combustion. Limited oxygen.

Produces CO and/or C (soot).
Less heat released per mole.
Causes safety problems (CO is poisonous).

Why alkanes are good fuels?

Burn cleanly when O₂ is plentiful.
Easily transported and stored as liquids or compressed gases.
Good energy density (kJ per g).
Wide availability via petroleum.

Worked qualitative. Why does a Bunsen burner produce a yellow, sooty flame when the air hole is closed? Limited oxygen → incomplete combustion → forms carbon (soot) and CO. Open the air hole → complete combustion → blue flame, hotter, cleaner.

Complete: + O₂ → CO₂ + H₂O.
Incomplete: + CO and/or C.
Major use: fuels.
Bunsen flame: blue (complete) / yellow (incomplete).

Substitution with halogens (UV)

Alkane + halogen + UV → halogenoalkane + HX.

Substitution reaction. A hydrogen atom is REPLACED by a halogen atom. Requires UV LIGHT to initiate.

General equation. $\text{C}_{n}\text{H}_{2n+2} + \text{X}_{2} \xrightarrow{\text{UV}} \text{C}_{n}\text{H}_{2n+1}\text{X} + \text{HX}.$

Worked. Methane + chlorine in UV light: $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}.$

The product chloromethane is a halogenoalkane.

One hydrogen (red) is replaced by a chlorine atom — substitution needs UV light to start.

Multiple substitutions are possible:

$\text{CH}_{3}\text{Cl} + \text{Cl}_{2} \to \text{CH}_{2}\text{Cl}_{2} + \text{HCl}$ (dichloromethane).
$\text{CH}_{2}\text{Cl}_{2} + \text{Cl}_{2} \to \text{CHCl}_{3} + \text{HCl}$ (trichloromethane / chloroform).
$\text{CHCl}_{3} + \text{Cl}_{2} \to \text{CCl}_{4} + \text{HCl}$ (tetrachloromethane).

Why UV? UV provides the energy to break Cl-Cl bond → forms Cl radicals that attack methane.

Worked. Ethane + chlorine in UV: $\text{C}_{2}\text{H}_{6} + \text{Cl}_{2} \to \text{C}_{2}\text{H}_{5}\text{Cl} + \text{HCl}.$

Cambridge tip. Always state UV LIGHT (or sunlight) as the condition for substitution. Without light, the reaction doesn't proceed at room temperature.

Substitution: H replaced by X (halogen).
Requires UV light.
Methane + Cl₂ → CH₃Cl + HCl.
Multiple substitutions possible.

How it’s examined

Alkanes appear every Paper 2 (3-4 marks: name, formula, combustion equation) and Paper 4 (4-6 marks: substitution mechanism, balanced equations). Examiner reports flag missing UV light as the reaction condition for substitution.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (13.4); 0620/42 Oct/Nov 2024 — Q23 (alkanes); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Alkanes

Step-by-step solutions to past-paper-style questions on alkanes, written exactly the way a tutor would explain them at the board.

1Substitution of methane
Extended• Adapted from 0620/42 May/Jun 2024 Q15• substitution
Question
Write the equation for the reaction of methane with chlorine in UV light, replacing one H.
Step-by-step solution
1. Step 1
  $\text{CH}_{4}$ + $\text{Cl}_{2}$ in UV → CH₃Cl + HCl.
  $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$
Answer
$\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$
Examiner tip
UV light is essential — provides the energy to start a free-radical chain reaction.
2Catalytic cracking
Extended• cracking
Question
Write a possible equation for cracking decane ( $\text{C}_{10}\text{H}_{22}$ ) into octane and one other product.
Step-by-step solution
1. Step 1
  Decane → smaller alkane + alkene.
  $\text{C}_{10}\text{H}_{22} \to \text{C}_{8}\text{H}_{18} + \text{C}_{2}\text{H}_{4}$
Answer
$\text{C}_{10}\text{H}_{22} \to \text{C}_{8}\text{H}_{18} + \text{C}_{2}\text{H}_{4}$ (octane + ethene)
3Properties of alkanes
Core• properties
Question
State three typical properties of small alkanes (≤ C5).
Step-by-step solution
1. Step 1
  Generally unreactive (no functional group).
2. Step 2
  Burn readily — used as fuels.
3. Step 3
  Insoluble in water; less dense than water.
Answer
Unreactive (apart from combustion), burn readily, insoluble in water.
4Combustion of butane
Extended• combustion
Question
Write the equation for the complete combustion of butane.
Step-by-step solution
1. Step 1
  $\text{C}_{4}\text{H}_{10}$ + O₂ → CO₂ + H₂O.
2. Step 2
  Balance.
  $2\text{C}_{4}\text{H}_{10} + 13\,\text{O}_{2} \to 8\text{CO}_{2} + 10\text{H}_{2}\text{O}$
Answer
$2\text{C}_{4}\text{H}_{10} + 13\,\text{O}_{2} \to 8\text{CO}_{2} + 10\text{H}_{2}\text{O}$

Key Formulae — Alkanes

The formulae you need to memorise for alkanes on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Cracking conditions
$\text{High temperature (} \sim 600\degree\text{C)} + \text{catalyst (e.g. Al}_{2}\text{O}_{3})$
When to use
Industrial cracking of long-chain alkanes.

Key Definitions and Keywords — Alkanes

Definitions to memorise and the exact keywords mark schemes credit for alkanes answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Alkane
Examiner keyword
Saturated hydrocarbon with general formula $\text{C}_{n}\text{H}_{2n+2}$ .
Substitution reaction
Examiner keyword
Reaction in which one atom or group is REPLACED by another (e.g. H by Cl in alkanes).
Cracking
Examiner keyword
Breaking long-chain hydrocarbons into smaller ones using heat and a catalyst — produces useful petrol-range alkanes and alkenes.

Common Mistakes and Misconceptions — Alkanes

The traps other students keep falling into on alkanes questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying alkanes undergo addition reactions
0620/42 — recurring
Why it happens
Confusing with alkenes.
How to avoid it
Alkanes: SUBSTITUTION (need UV). Alkenes: ADDITION (no UV needed).
✕Forgetting the UV condition for substitution
Why it happens
Speed.
How to avoid it
Always include 'UV' or 'sunlight' over the arrow for halogenation of alkanes.
✕Producing only smaller alkanes from cracking
Why it happens
Forgetting alkenes.
How to avoid it
Cracking ALWAYS produces an alkane + at least one alkene (otherwise H atoms don't balance).

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Alkanes — frequently asked questions

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Alkanes

Short Study Notes in the form of Slides

Short Study Notes — Alkanes

Detailed Notes

What you’ll learn

How it’s examined

1Substitution of methane

2Catalytic cracking

3Properties of alkanes

4Combustion of butane

Cracking conditions

Alkane

Substitution reaction

Cracking

✕Saying alkanes undergo addition reactions

✕Forgetting the UV condition for substitution

✕Producing only smaller alkanes from cracking

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Alkanes — frequently asked questions