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.

Homologous Series

A homologous series is a group or family of organic compounds that share the same functional group, exhibit similar chemical properties, and can be represented by a general molecular formula. Consecutive members of this series differ by a $-CH_2-$ unit in their molecular formula and by $14\text{ u}$ in their molecular mass.

Chemical Formulae of Two Members

Taking the alkane homologous series as an example:

1. Methane: $CH_4$
2. Ethane: $C_2H_6$

(Alternatively, for the alcohol series: Methanol ($CH_3OH$) and Ethanol ($C_2H_5OH$))

Parts Determining the Properties

• (i) Physical Properties: Determined by the alkyl group / carbon chain length (size of the molecule). As the molecular mass increases down the series, physical properties like melting point, boiling point, and density increase systematically due to stronger intermolecular forces.
• (ii) Chemical Properties: Determined by the functional group (e.g., $-OH$, $-COOH$, or the nature of bonds like double/triple bonds). Since all members of a homologous series share the same functional group, they undergo identical chemical reactions.