---
title: "Alkanes and Alkenes: The First Homologous Series"
description: "Alkanes and alkenes are families whose members differ by one carbon at a time and share a general formula. The double bond in an alkene makes it react in ways an alkane will not."
canonical: https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL
source: https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL.md
retrieved: 2026-09-12
---

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# Alkanes and Alkenes: The First Homologous Series

Alkanes and alkenes are families whose members differ by one carbon at a time and share a general formula. The double bond in an alkene makes it react in ways an alkane will not.

Subject: Science · Area: Matter & Materials · Ages 15 to 16
Page: https://lightmysky.com/learn/science/alkanes-and-alkenes-the-first-homologous-series-mt_mkLJDtBanL

## Ready when they can

- Writes the name and formula of the first four alkanes and the first three alkenes
- States the general formula of each series and explains what makes a set of compounds a homologous series
- Describes the bromine water test and the result it gives for each family
- Writes the products of complete and of incomplete combustion of a named hydrocarbon

## Lesson: Two families, one double bond apart

Alkanes are carbon chains with single bonds only. The first four are methane CH4, ethane C2H6, propane C3H8 and butane C4H10. They share the general formula CnH2n+2. A set like this is a homologous series: one general formula, with each member differing from the next by CH2, and steady trends in properties.

Alkenes carry one C=C double bond, and that changes everything. The first three are ethene C2H4, propene C3H6 and butene C4H8. Their general formula is CnH2n. The double bond opens up to addition reactions, so alkenes react in ways alkanes will not.

**Example.** Shake each gas with orange bromine water. Propene turns it colourless, because the double bond adds bromine across itself. Propane leaves it orange, because single bonds ignore bromine in the cold. A vanishing colour proves C=C.

**Tip.** Burning sorts them too. Complete combustion in plenty of oxygen gives carbon dioxide and water. A short air supply gives carbon monoxide, soot and water instead. Alkanes also swap hydrogens for halogens one by one in ultraviolet light.

**Recap.** Single-bonded alkanes follow CnH2n+2, double-bonded alkenes follow CnH2n, and bromine water tells them apart.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Types of Chemical Reaction](https://lightmysky.com/learn/science/types-of-chemical-reaction-mt_v0K6GRi4ZL)
- [Bond Energies and the Overall Energy Change](https://lightmysky.com/learn/science/bond-energies-and-the-overall-energy-change-mt_VI0_1ogIOy)

## Opens up

- [Electrophilic Addition to Alkenes](https://lightmysky.com/learn/science/electrophilic-addition-to-alkenes-mt_1RB5nj8zR8)
- [Isomerism: Same Formula, Different Molecule](https://lightmysky.com/learn/science/isomerism-same-formula-different-molecule-mt_DvD0coeKZe)
- [Crude Oil, Fractional Distillation and Cracking](https://lightmysky.com/learn/science/crude-oil-fractional-distillation-and-cracking-mt_Jg3O4EQ9Vz)
- [Naming Organic Compounds and the Formulas That Show Structure](https://lightmysky.com/learn/science/naming-organic-compounds-and-the-formulas-that-show-structure-mt_LCqg41gslT)
- [Elimination and Choosing Between Two Pathways](https://lightmysky.com/learn/science/elimination-and-choosing-between-two-pathways-mt_tRj_kiSS51)