2.1Nature’s Chemistry
Homologous Series — SQA National 5 Chemistry — SQA National 5 Chemistry Revision Notes & Topical Questions
5 min read 2 parts Lesson 1 of 3
Section 1 of 2Alkanes, alkenes and cycloalkanes
| Homologous Series | General Formula | Saturation | Functional Feature | First 3 Members |
|---|---|---|---|---|
| Alkanes | CnH2n+2 | Saturated (single C-C bonds only) | Unreactive, undergo combustion | Methane (CH4), Ethane (C2H6), Propane (C3H8) |
| Alkenes | CnH2n | Unsaturated (contains C=C double bond) | Reactive, undergo addition reactions | Ethene (C2H4), Propene (C3H6), Butene (C4H8) |
| Cycloalkanes | CnH2n | Saturated (closed carbon ring, single bonds) | Ring structure | Cyclopropane (C3H6), Cyclobutane (C4H8), Cyclopentane (C5H10) |
- Trends in physical propertiesAs carbon chain length increases, melting and boiling points increase because intermolecular forces between larger molecules become stronger.
- Bromine water test for unsaturation:
- Alkenes immediately decolourise bromine solution from brown-orange to colourless by an addition reaction: C2H4 + Br2 → C2H4Br2 (1,2-dibromoethane).
- Saturated hydrocarbons (alkanes and cycloalkanes) do not react; bromine remains brown-orange.
Section 2 of 2Isomers and systematic naming
1Isomers3 points
- Compounds with the same molecular formula but different structural formulae.
- May have different physical properties (e.g. branched isomers have lower boiling points due to less surface contact between molecules).
- Alkenes and cycloalkanes with the same number of carbon atoms are isomers of each other (e.g. propene and cyclopropane both have formula C3H6).
2IUPAC naming rules for branched hydrocarbons4 points
- Find and name the longest continuous carbon chain (stem name).
- Number the carbon chain from the end that gives branches (or double bonds) the lowest possible numbers.
- Identify branch groupsmethyl (-CH3), ethyl (-C2H5).
- Use prefixes di-, tri-, tetra- if multiple identical branches are present.
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🌳 Student Search Tree & Frequently Asked Exam Questions: homologous series
Step-by-step problem-solving guide, mark scheme criteria, and experimental error analysis curated by Dr. Khaled Dabour (DABOURHUB).
1. Identify all given values and convert non-standard units, 2. State the primary formula or physiological/chemical principle, 3. Calculate to 3 significant figures and clearly state units.
Avoid confusing independent vs dependent variables in practical questions. In calculation questions, double-check percentage change formula: (change / original) × 100.
Where can I find homologous series past paper questions by topic with the mark scheme?
On DABOURHUB every homologous series question is grouped by syllabus point and shown with the mark-scheme wording, so you can see exactly which sentence earns the mark instead of scrolling through whole year papers.
What is the most common homologous series exam question and why do students lose marks on it?
The repeated pattern on IGCSE papers is a short stem that asks you to apply homologous series and then justify it with a command word (explain, suggest, compare). Marks are lost by stating the fact without linking it back to the wording of the question, so each homologous series item here is written as a full-mark answer.
Is there an alternative to Save My Exams or Physics and Maths Tutor for homologous series?
Yes: the homologous series notes, topical questions and MCQ quizzes on this site follow the same syllabus headings as the official specification, and they open straight from the search result with no download step.
How do I revise homologous series in two weeks before the exam?
Three passes: read the homologous series notes once, then do the topical questions and mark them against the mark scheme, then redo only the ones you missed. The quiz on the page gives instant feedback so the third pass takes minutes rather than an evening.