What is meant by homologous series of organic compounds? Write the chemical formulae of two members of a homologous series and state which part determines the (i) physical properties, (ii) chemical properties, of these compounds.

The series which consists of similar structures and similar chemical properties in which the successive compounds differs by a CH₂ group is termed as homologous series.

The two characteristics of homologous series are as follows:

1. The members of the homologous series consists of same general formula such as Alkanes have general formula C_nH_2n+2

2. The two adjacent homologues have a difference of molecular mass of 14 u.

The general formula for the alkenes is: C_nH_2n. The members of the series of alkenes is :

1^st member is C₂H₆ (Ethene)

2nd member is C₃H₉ (Propene)

(i) The number of carbon atoms decides the physical properties. Increase in Carbon atoms in the homologus series results in increase in the melting points, boiling points and density.

(ii) The compounds of homologues series have same chemical properties due to same basic structure of members of homologus series.