# A student is given samples of three organic compounds: compound A (C₂H₆), compound B (C₄H₁₀), and compound C (C₃H₈). All three compounds belong to the same homologous series. Explain what is meant by the term 'homologous series' and describe two trends in physical properties that would be observed as the number of carbon atoms increases across these three compounds.

> Edexcel A-Level Chemistry (9CH0) — 6.1 Introduction to organic chemistry · Explain · 5 marks

> Organic compounds can be grouped into homologous series. Compound A has the molecular formula C₂H₆, compound B has the molecular formula C₄H₁₀, and compound C has the molecular formula C₃H₈.

## Mark scheme (5 marks)

1. A homologous series is a family/group of compounds that share the same general formula
2. Members of a homologous series differ by one CH₂ unit (from one member to the next)
3. Members of a homologous series have similar chemical properties
4. Boiling point increases as the number of carbon atoms increases
5. Viscosity increases (the compounds become less easy to pour / flow) as the number of carbon atoms increases

## Key terms

- [homologous series](https://www.gradenine.co.uk/glossary/homologous-series)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-student-is-given-samples-of-1dba31ed) · Published by Druglandscape Ltd.