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 alkanes with alkenes. How to avoid it: Alkanes are saturated → SUBSTITUTION (need UV). Alkenes have a C=C → ADDITION (no UV needed). Source: 0620/42 — recurring

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

Why it happens: Rushing, or treating it like any other reaction. How to avoid it: Always write 'UV' (or sunlight) over the arrow for the halogenation of an alkane — without it there is no reaction.

Why is "Giving CO₂ as a product of incomplete combustion." a common mistake in Alkanes?

Why it happens: Treating all burning the same and forgetting the oxygen supply matters. How to avoid it: Plentiful O₂ → CO₂ + H₂O (complete). Limited O₂ → CO and/or C + H₂O (incomplete).

Why is "Leaving a fractional $\text{O}_{2}$ coefficient (e.g. $6.5\,\text{O}_{2}$) in a final equation." a common mistake in Alkanes?

Why it happens: Stopping after balancing C and H. How to avoid it: If oxygen comes out as a half, multiply the WHOLE equation by 2 to give whole-number coefficients.

Cambridge IGCSE Chemistry (0620)

Alkanes

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Short Notes - Alkanes

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Detailed Study Notes

Detailed notes on Organic Chemistry for Cambridge IGCSE Chemistry, covering key concepts, explanations, examples, and exam-focused revision points.

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.

At a glance

Saturated — only single C-C bonds.
General formula: $\text{C}_{n}\text{H}_{2n+2}$ .
First six: methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), hexane (C₆H₁₄).
Generally UNREACTIVE (saturated).
Two key reactions: COMBUSTION (with oxygen) and SUBSTITUTION (with halogens, in UV light).
Substitution: $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{UV} \text{CH}_{3}\text{Cl} + \text{HCl}$ .

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.

Quick recap

Saturated hydrocarbons.
$\text{C}_{n}\text{H}_{2n+2}$ .
First 6: meth, eth, prop, but, pent, hex.
Combust: + O₂ → CO₂ + H₂O.
Substitute: + halogen + UV → halogenoalkane + HX.
Otherwise unreactive.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Alkane — saturated hydrocarbon with general formula $\text{C}_{n}\text{H}_{2n+2}$ .
Saturated — containing only single bonds between carbon atoms.
Substitution reaction — replacing one atom in a molecule with another.
Halogenoalkane — alkane in which at least one H has been replaced by a halogen.

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.

Take this whole topic with you

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.

1General formula and saturation
Getting started• general formula, saturated
Question
Propane has 3 carbon atoms. Use the general formula for alkanes to work out its molecular formula, and state what 'saturated' means.
Step-by-step solution
1. Step 1
  Alkanes have the general formula $\text{C}_{n}\text{H}_{2n+2}$ . For propane, $n = 3$ .
  $\text{C}_{n}\text{H}_{2n+2},\quad n = 3$
2. Step 2
  Substitute: hydrogens $= (2 \times 3) + 2 = 8$ , so the formula is $\text{C}_{3}\text{H}_{8}$ .
  $\text{C}_{3}\text{H}_{(2\times 3)+2} = \text{C}_{3}\text{H}_{8}$
3. Step 3
  'Saturated' means every bond between the carbon atoms is a SINGLE C–C bond, so the molecule contains the maximum possible number of hydrogen atoms (there are no C=C double bonds).
Answer
$\text{C}_{3}\text{H}_{8}$ . Saturated = only single C–C bonds, so the molecule holds the maximum number of hydrogen atoms.
Examiner tip
Learn the first four by name: methane $\text{CH}_{4}$ , ethane $\text{C}_{2}\text{H}_{6}$ , propane $\text{C}_{3}\text{H}_{8}$ , butane $\text{C}_{4}\text{H}_{10}$ .
2Typical properties of alkanes
Getting started• properties
Question
State three typical properties of small alkanes (≤ C5).
Step-by-step solution
1. Step 1
  Generally unreactive — they have no reactive functional group (no C=C double bond).
2. Step 2
  Burn readily in a plentiful supply of air — which is why they are used as fuels.
3. Step 3
  Insoluble in water and less dense than water; the small ones are gases at room temperature.
Answer
Unreactive (apart from combustion and substitution), burn readily as fuels, and are insoluble in water.
3Complete combustion of butane
Building confidence• combustion
Question
Write a balanced equation for the complete combustion of butane ( $\text{C}_{4}\text{H}_{10}$ ).
Step-by-step solution
1. Step 1
  Complete combustion (plenty of oxygen) of any hydrocarbon gives carbon dioxide and water only.
  $\text{C}_{4}\text{H}_{10} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}$
2. Step 2
  Balance C and H first: 4 carbons → $4\text{CO}_{2}$ ; 10 hydrogens → $5\text{H}_{2}\text{O}$ .
  $\text{C}_{4}\text{H}_{10} + \text{O}_{2} \to 4\text{CO}_{2} + 5\text{H}_{2}\text{O}$
3. Step 3
  Now balance O: the right side has $(4\times 2) + 5 = 13$ oxygen atoms, i.e. $6.5\,\text{O}_{2}$ . Double everything to remove the fraction.
  $2\text{C}_{4}\text{H}_{10} + 13\,\text{O}_{2} \to 8\text{CO}_{2} + 10\text{H}_{2}\text{O}$
Answer
$2\text{C}_{4}\text{H}_{10} + 13\,\text{O}_{2} \to 8\text{CO}_{2} + 10\text{H}_{2}\text{O}$
Examiner tip
Balance C and H first, oxygen last. Doubling the whole equation is the standard way to clear the $6.5\,\text{O}_{2}$ fraction.
4Incomplete combustion
Building confidence• combustion, incomplete
Question
Methane burns in a LIMITED supply of oxygen. Name the carbon-containing products and write one possible balanced equation.
Step-by-step solution
1. Step 1
  In a limited supply of oxygen there is not enough to oxidise all the carbon fully, so incomplete combustion produces carbon monoxide (CO) and/or carbon (soot), as well as water.
2. Step 2
  One possible equation gives carbon monoxide as the carbon-containing product. Balance C, H, then O.
  $2\text{CH}_{4} + 3\text{O}_{2} \to 2\text{CO} + 4\text{H}_{2}\text{O}$
3. Step 3
  If even less oxygen is present, soot (carbon) is also formed: $\text{CH}_{4} + \text{O}_{2} \to \text{C} + 2\text{H}_{2}\text{O}$ .
  $\text{CH}_{4} + \text{O}_{2} \to \text{C} + 2\text{H}_{2}\text{O}$
Answer
Carbon monoxide (CO) and carbon (soot). e.g. $2\text{CH}_{4} + 3\text{O}_{2} \to 2\text{CO} + 4\text{H}_{2}\text{O}$ .
Examiner tip
Incomplete combustion = limited oxygen → CO and/or C plus water. CO is toxic; carbon shows as a smoky/yellow flame.
5Substitution of methane with chlorine
Stretch• Adapted from 0620/42 May/Jun 2024 Q15• substitution, photochemical
Question
Write the equation for the reaction of methane with chlorine in the presence of UV light, replacing one hydrogen atom, and explain why this is called a substitution reaction.
Step-by-step solution
1. Step 1
  UV (ultraviolet) light supplies the energy needed to start this photochemical reaction.
2. Step 2
  One hydrogen atom of methane is REPLACED by a chlorine atom, forming chloromethane and hydrogen chloride.
  $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$
3. Step 3
  It is called substitution because an atom (H) is swapped for another atom (Cl) — one at a time — unlike addition in alkenes where no atoms are lost.
Answer
$\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$ . A hydrogen atom is replaced (substituted) by a chlorine atom.
Examiner tip
UV light is essential — without it there is no reaction. Only ONE H is replaced per step, releasing one HCl molecule each time.
6Why alkanes are unreactive (compared with alkenes)
Stretch• reactivity, comparison
Question
Explain why alkanes are generally unreactive, and contrast their reaction with chlorine with that of alkenes.
Step-by-step solution
1. Step 1
  Alkanes are SATURATED — they contain only single C–C and C–H bonds, which are strong and difficult to break. There is no C=C double bond to act as a reactive site.
2. Step 2
  The C–H and C–C bonds are also non-polar, so alkanes are not attacked by common reagents and do not react easily under normal conditions.
3. Step 3
  With chlorine, an alkane undergoes SUBSTITUTION and needs UV light: $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$ .
  $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$
4. Step 4
  An alkene (e.g. ethene) has a reactive C=C double bond, so it undergoes ADDITION with chlorine — without UV — and no HCl is produced: $\text{C}_{2}\text{H}_{4} + \text{Cl}_{2} \to \text{C}_{2}\text{H}_{4}\text{Cl}_{2}$ .
  $\text{C}_{2}\text{H}_{4} + \text{Cl}_{2} \to \text{C}_{2}\text{H}_{4}\text{Cl}_{2}$
Answer
Alkanes are unreactive because they are saturated, with only strong, non-polar single bonds and no C=C. They react with chlorine by substitution (UV needed, HCl made); alkenes react by addition across the C=C (no UV, no HCl).
Examiner tip
The discriminator: substitution (alkanes, UV, gives a halogenoalkane + HCl) vs addition (alkenes, C=C, gives a single dihalogenoalkane).

Model Answers — Alkanes

High-scoring sample answers for alkanes on the Cambridge IGCSE 0620 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 4 short-answer style1 mark
State the general formula of the alkanes. (1 mark)
Model answer
$\text{C}_{n}\text{H}_{2n+2}$
Why this scores
One mark for the exact general formula. The ' $+2$ ' is essential — $\text{C}_n\text{H}_{2n}$ is the alkene formula.
Question 2
Paper 4 structured style2 marks
Alkanes are described as saturated hydrocarbons. State what is meant by saturated and by hydrocarbon. (2 marks)
Model answer
Saturated means the molecule contains only single carbon–carbon (C–C) bonds, so it holds the maximum number of hydrogen atoms (no C=C double bonds). Hydrocarbon means a compound containing only the elements hydrogen and carbon.
Why this scores
One mark for 'only single C–C bonds / no double bonds', one mark for 'compound of hydrogen and carbon only'.
Question 3
Paper 4 structured style3 marks
Write a balanced symbol equation for the complete combustion of methane, $\text{CH}_{4}$ . (3 marks)
Model answer
$\text{CH}_{4} + 2\text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$
Why this scores
Marks for: correct products (CO₂ and H₂O); correct formulae; correctly balanced (note the $2\text{O}_{2}$ and $2\text{H}_{2}\text{O}$ ). Complete combustion means a plentiful supply of oxygen.
Question 4
Paper 4 (Extended) structured style4 marks
Explain why incomplete combustion of an alkane is dangerous, naming the products formed. (4 marks)
Model answer
Incomplete combustion happens when there is a limited (insufficient) supply of oxygen. The carbon is not fully oxidised, so the products include carbon monoxide (CO) and/or carbon (soot), together with water. Carbon monoxide is a toxic gas: it binds to haemoglobin in the blood and reduces the amount of oxygen the blood can carry, which can be fatal. It is also colourless and odourless, so it cannot easily be detected. The soot (carbon) produced can blacken surfaces and block appliances.
Why this scores
Four marks across: limited oxygen supply; carbon monoxide (and/or carbon) produced; CO is toxic / reduces oxygen transport by haemoglobin; CO is colourless and odourless so hard to detect.
Question 5
Paper 4 (Extended) structured style5 marks
Methane reacts with chlorine to form chloromethane. Write the equation for the reaction, state the essential condition, and explain why this is a substitution reaction. (5 marks)
Model answer
The equation is $\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$ . The essential condition is ultraviolet (UV) light (sunlight), which supplies the energy to start this photochemical reaction. It is a substitution reaction because one hydrogen atom of the methane is replaced by a chlorine atom — atoms are swapped rather than added across a bond. Because a hydrogen is removed, a molecule of hydrogen chloride (HCl) is also formed. Only one hydrogen is replaced at a time, so further substitution could continue with more chlorine.
Why this scores
Five marks: correct balanced equation; UV light condition; 'photochemical'; one H replaced by Cl (definition of substitution); HCl also formed / one H at a time.
Question 6
Paper 4 (Extended) structured style6 marks
Explain why alkanes are generally unreactive, and describe how their reaction with bromine differs from that of alkenes. (6 marks)
Model answer
Alkanes are saturated, meaning they contain only single C–C and C–H bonds with no C=C double bond to act as a reactive site. These bonds are strong and non-polar, so they are hard to break and are not readily attacked by other reagents — this is why alkanes are generally unreactive under normal conditions. With bromine, an alkane reacts only by substitution, and only in the presence of UV light: a hydrogen atom is replaced by a bromine atom and hydrogen bromide (HBr) is produced, e.g. $\text{CH}_{4} + \text{Br}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Br} + \text{HBr}$ . The reaction is slow and the orange bromine colour fades only in UV light. An alkene has a reactive C=C double bond, so it reacts with bromine by addition — without UV light and rapidly, decolourising bromine water at once, with no HBr formed, e.g. $\text{C}_{2}\text{H}_{4} + \text{Br}_{2} \to \text{C}_{2}\text{H}_{4}\text{Br}_{2}$ . This difference is the basis of the bromine-water test that distinguishes alkanes from alkenes.
Why this scores
Six marks across: saturated / only single bonds / no C=C; strong, non-polar bonds → unreactive; alkane = substitution needing UV, HBr formed; alkene = addition across C=C, no UV; alkene reaction fast; link to the bromine-water test (alkene decolourises quickly, alkane only in UV).

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.

General formula of alkanes
$\text{C}_{n}\text{H}_{2n+2}$
$n$
number of carbon atoms in the molecule
When to use
Work out the molecular formula of any alkane from its number of carbons (e.g. $n=4 \Rightarrow \text{C}_4\text{H}_{10}$ ).
Substitution with chlorine (photochemical)
$\text{CH}_{4} + \text{Cl}_{2} \xrightarrow{\text{UV}} \text{CH}_{3}\text{Cl} + \text{HCl}$
When to use
Halogenation of an alkane — one H replaced by one halogen atom per step; requires UV light.

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
A saturated hydrocarbon with the general formula $\text{C}_{n}\text{H}_{2n+2}$ (e.g. methane, ethane, propane, butane).
Saturated
Examiner keyword
Containing only single carbon–carbon (C–C) bonds, so the molecule holds the maximum possible number of hydrogen atoms (no C=C double bonds).
Substitution reaction
Examiner keyword
A reaction in which one atom (or group) is REPLACED by another — e.g. an H atom of an alkane replaced by a Cl atom in UV light.
Incomplete combustion
Examiner keyword
Burning in a limited supply of oxygen, producing carbon monoxide (CO) and/or carbon (soot) plus water, instead of carbon dioxide.

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 alkanes with alkenes.
How to avoid it
Alkanes are saturated → SUBSTITUTION (need UV). Alkenes have a C=C → ADDITION (no UV needed).
✕Forgetting the UV condition for substitution.
Why it happens
Rushing, or treating it like any other reaction.
How to avoid it
Always write 'UV' (or sunlight) over the arrow for the halogenation of an alkane — without it there is no reaction.
✕Giving CO₂ as a product of incomplete combustion.
Why it happens
Treating all burning the same and forgetting the oxygen supply matters.
How to avoid it
Plentiful O₂ → CO₂ + H₂O (complete). Limited O₂ → CO and/or C + H₂O (incomplete).
✕Leaving a fractional $\text{O}_{2}$ coefficient (e.g. $6.5\,\text{O}_{2}$ ) in a final equation.
Why it happens
Stopping after balancing C and H.
How to avoid it
If oxygen comes out as a half, multiply the WHOLE equation by 2 to give whole-number coefficients.

Alkanes — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Alkanes

Short Notes - Alkanes

Detailed Study Notes

At a glance

What you’ll learn

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1General formula and saturation

2Typical properties of alkanes

3Complete combustion of butane

4Incomplete combustion

5Substitution of methane with chlorine

6Why alkanes are unreactive (compared with alkenes)

General formula of alkanes

Substitution with chlorine (photochemical)

Alkane

Saturated

Substitution reaction

Incomplete combustion

✕Saying alkanes undergo addition reactions.

✕Forgetting the UV condition for substitution.

✕Giving CO₂ as a product of incomplete combustion.

✕Leaving a fractional O2\text{O}_{2}O2​ coefficient (e.g. 6.5 O26.5\,\text{O}_{2}6.5O2​) in a final equation.

Alkanes — frequently asked questions

✕Leaving a fractional $\text{O}_{2}$ coefficient (e.g. $6.5\,\text{O}_{2}$ ) in a final equation.