What is a homologous series? State two characteristics that all members of a homologous series share.
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer
A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, with each successive member differing by a –CH₂– unit (molecular mass difference of 14 u).
Two characteristics shared by all members:
- Similar chemical properties — Since chemical properties are determined solely by the functional group, all members show similar chemical reactions (e.g., all alcohols like CH₃OH, C₂H₅OH show similar reactions).
- Gradation in physical properties — As molecular mass increases, physical properties such as melting point, boiling point, and solubility show a regular gradation.
Source: Chapter 4, Section 4.2.4 — Homologous Series
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Explanation
- The definition (1 mark) must include two key ideas: same functional group and successive members differ by –CH₂–.
- For the two characteristics (1 mark each), examiners expect: similar chemical properties and gradation in physical properties. Giving an example (like the alcohols) strengthens the answer but is not compulsory.
- Avoid writing that physical properties are the same — they are similar in trend/gradation, not identical.