Who this is for: Cambridge IGCSE Coordinated Science (0654) students who want homologous series — general formulas, functional groups and property trends — to become reliable marks instead of memorised lists without understanding.
What query it owns: how to understand and revise homologous series in Cambridge IGCSE Coordinated Science.
Why this is safe: this page owns the homologous-series revision-guide angle, while Tutopiya’s Homologous Series subtopic page owns the learning resource and the free Homologous Series quiz owns the practice.

A homologous series is a family of organic compounds with the same functional group and similar chemical properties. Each member differs from the next by a –CH₂– group. Cambridge IGCSE Coordinated Science (0654) tests the definition, general formulas for alkanes, alkenes and alcohols, and trends such as increasing boiling point with chain length. This guide covers the syllabus definitions, comparison tables examiners expect, and the question types that appear every year.

Key takeaways

• Members of a homologous series share the same functional group and show a gradual trend in physical properties.
• Successive members differ by a –CH₂– unit (relative molecular mass increases by 14).
• Alkanes: general formula CₙH₂ₙ₊₂; alkenes: CₙH₂ₙ; alcohols: CₙH₂ₙ₊₁OH.
• Boiling point increases as carbon chain length increases (stronger intermolecular forces).
• Compounds in the same series have similar chemical reactions because they share the same functional group.

What is a homologous series in Cambridge IGCSE Coordinated Science?

Organic chemistry organises compounds into families. All alkanes, for example, contain only C–C and C–H single bonds and react similarly — they burn and undergo substitution. Each homologous series is defined by its functional group: alkenes have C=C, alcohols have –OH. Knowing the general formula lets you predict molecular formulas and identify which series a compound belongs to.

Read the full notes on Tutopiya’s Homologous Series subtopic page before attempting questions.

The core ideas you must master

Idea	What it means	How the exam uses it
Same functional group	Defines chemical behaviour	”State the functional group in alkenes.”
–CH₂– difference	Each member adds one CH₂ unit	”How does propane differ from butane?”
General formula	Predicts molecular formula	”Give the general formula of alkanes.”
Trend in b.p.	Longer chain → higher b.p.	”Explain why butane boils above propane.”
Similar reactions	Same functional group → same chemistry	”Why do all alkanes burn?”

Homologous series at IGCSE — comparison table

Series	Functional group	General formula	Example	Suffix
Alkanes	C–C, C–H (saturated)	CₙH₂ₙ₊₂	Ethane (C₂H₆)	-ane
Alkenes	C=C (double bond)	CₙH₂ₙ	Ethene (C₂H₄)	-ene
Alcohols	–OH (hydroxyl)	CₙH₂ₙ₊₁OH	Ethanol (C₂H₅OH)	-ol

Trends in physical properties

Trend	Explanation	Exam example
Boiling point ↑ with chain length	Larger molecules → stronger intermolecular forces	”Explain why pentane boils above butane.”
Volatility ↓ with chain length	Stronger forces → less easily vaporised	”Which is more volatile: methane or butane?”
Mr increases by 14 per –CH₂–	CH₂ relative mass = 12 + 2 = 14	”Find Mr difference between propane and butane.”

Features of a homologous series — checklist

1. Same functional group in every member.
2. Each successive member differs by –CH₂–.
3. Same general formula.
4. Gradual change in physical properties (b.p., melting point).
5. Similar chemical properties and reactions.

Homologous series in past-paper wording

Command word	What the question wants	Typical stem
Define	Homologous series definition	”Define homologous series.”
State	General formula	”State the general formula of alkenes.”
Explain	Trend in property	”Explain why boiling point increases down a series.”
Identify	Series from formula	”Identify the homologous series of C₃H₈.”
Compare	Two members of a series	”Compare ethane and propane.”

Worked exam-style stems

1. “Define homologous series.” A family of organic compounds with the same functional group, similar chemical properties, and each member differing from the next by a –CH₂– group. Reward: functional group + similar properties + –CH₂– difference.
2. “State the general formula of alcohols and give one example.” CₙH₂ₙ₊₁OH; example: ethanol (C₂H₅OH). Reward: correct formula + named example.
3. “Explain why the boiling point of alkanes increases as chain length increases.” Longer chains have more electrons → stronger intermolecular (van der Waals) forces → more energy needed to separate molecules → higher boiling point. Reward: longer chain + stronger forces + higher b.p.

Practise on the Homologous Series quiz.

How homologous series connect to the syllabus

Homologous series underpin Alkanes, Alkenes, Names of Compounds and Fuels. The Cambridge IGCSE Coordinated Science resource hub links every Organic Chemistry subtopic.

Common mistakes students make

• Confusing general formulas: CₙH₂ₙ₊₂ (alkanes) vs CₙH₂ₙ (alkenes).
• Saying members differ by CH₄ instead of –CH₂–.
• Stating all organic compounds are in one homologous series (each functional group defines a separate series).
• Explaining boiling-point trends with covalent bond strength (it is intermolecular forces).
• Forgetting that similar chemical properties come from the same functional group.

When you need more support

If homologous-series questions keep costing marks, work through the Homologous Series quiz, then get help from a Cambridge IGCSE Coordinated Science tutor.

Frequently asked questions

What is a homologous series? A family of organic compounds with the same functional group, similar chemical properties, and each member differing by a –CH₂– group.

What is the general formula of alkanes? CₙH₂ₙ₊₂.

Why do boiling points increase down a homologous series? Longer carbon chains have stronger intermolecular forces, requiring more energy to overcome.

How do I revise homologous series effectively? Learn the definition, three general formulas, functional groups and boiling-point trend, then take the quiz.

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