A series of compounds in which the same functional group substitutes in a carbon chain is called homologous series . For Example = CH₃OH , C₂H₅OH and C₃H₇OH.

The difference between successive members of a homologous series is equivalent to CH₂ or 14u in terms of molecular mass ( atomic mass of C is 12u and that of H is 1u). 

Melting points and boiling points increase with increasing molecular mass in a homologous series .

Chemical properties which are determined by the functional group remain similar in a homologous series . 
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Groups consisting of similiar funtional groups and gradual increase in number of carbons is known as Homologous series.