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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
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.
Model answer (5 marks)
A homologous series is a group of compounds that share the same general formula and have similar chemical properties. Members differ by one CH₂ unit.
Two physical‑property trends:
1. Boiling point rises as the carbon chain length increases.
2. Viscosity increases, making the liquids harder to pour, as the number of carbon atoms grows.
Two physical‑property trends:
1. Boiling point rises as the carbon chain length increases.
2. Viscosity increases, making the liquids harder to pour, as the number of carbon atoms grows.
Examiner tips
- Define the series and CH₂ difference for full marks; mention similar chemical properties. Show the two trends clearly, naming boiling point and viscosity. Use correct chemical terminology and UK spelling.
Common mistakes
- Confusing the series with a single compound; not stating the CH₂ difference. Listing a property that does not change (e.g. density) or omitting viscosity.
Mark scheme (5 marks)
- A homologous series is a family/group of compounds that share the same general formula
- Members of a homologous series differ by one CH₂ unit (from one member to the next)
- Members of a homologous series have similar chemical properties
- Boiling point increases as the number of carbon atoms increases
- Viscosity increases (the compounds become less easy to pour / flow) as the number of carbon atoms increases
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Introduction to organic chemistry questions
- Pentane (C₅H₁₂) is a member of the alkane homologous series. Explain what is mea…
- Crude oil is a mixture of hydrocarbons. Fractional distillation is used to separ…
- Crude oil is a finite resource formed from the remains of ancient organisms. It …
- Methane, ethane, propane and butane are all members of the alkane homologous ser…