Homologous series
- CBSE · NCERT Class 10, Ch 4 Carbon and its Compounds
A homologous series is a family of carbon compounds with the same functional group, each member differing from the next by one CH₂ unit.
The functional group decides how a compound reacts. So members of one series have very similar chemical properties. Their physical properties, like melting point, change step by step as the chain grows.
The alcohols are one series: CH₃OH, C₂H₅OH, C₃H₇OH, C₄H₉OH. Each has the –OH group. Each is one –CH₂– longer than the one before.
A small example
How much heavier is each member than the last? One CH₂ unit has 1 carbon and 2 hydrogen atoms. Carbon is 12 u and hydrogen is 1 u. So the step is 12 + 2 × 1 = 14 u. Methane, CH₄, is 16 u. Ethane, C₂H₆, is 24 + 6 = 30 u. The difference is 14 u.
Where marks go
Question 26 of CBSE's 2026-27 Science sample paper asks for the name and formula of the next homologue of X, which is ethanol. The 2026-27 Science marking scheme gives half a mark for the name, propanol, and half for the formula, C₃H₇OH.
So the question asks for two things, and each carries its own half mark. Write both. Read how step marking works.
Sources
Facts last checked against these sources on 30 September 2026.
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