A homologous series may be defined as a family of organic compounds having the same functional group,and the successive(adjacent) members of which differ by a CH _{2} unit or 14 mass units.

Two consecutive members of homologous series of Aldehydes: CH _{3} CHO and C _{2} H _{5} CHO.

A carbon compound is made up of two parts: R-F,where R is the hydrocarbon part and F is the functional group part.
So,this R (hydrocarbon) part determines the physical properties of a compound and the  F (functional group) part determines the chemical properties of the compound.  
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