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MODEL PAPER - 7 [2026] — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceMODEL PAPER - 7 [2026]3 Marks
Question
What is a homologous series? Find the difference in molecular mass between the two consecutive members of a homologous series. State how in a homologous series of carbon compounds the following properties vary with increase in molecular mass:
a. Melting and boiling points
b. Chemical properties
✓

Answer

a. A group of organic compounds having similar structures and similar properties and whose successive members differ by $-{CH}_{2}$ group is called a homologous series.
Example : All the alkanes have a similar structure with single covalent bonds and show similar chemical properties.
Therefore, they form a homologous series of alkanes. The first six members of this homologous series are given below :
Name of the alkaneMolecular formulaName of the alkane
Methane$CH _4$Butane
Ethane$C _2 H _6$Petane
Propane$C _3 H _{11}$Hexane
The general formula of the homologous series of alkanes is $C _n H _{2 n+2}$.
b. i. Molecular formula of any two adjacent homologues differ from each other by 14 units with regard to molecular mass.
ii. Molecular formulae of any two adjacent homologues differ by $- CH _2$ group, i.e., they differ by one carbon and two hydrogen atoms.

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