Explain the term 'Homologous series'. State the general characteristics of members of the series with special reference to molecular mass or molecular formula.

A Homologous series is a group of organic compounds having a similar structure and similar chemical properties in which the successive compounds differ by a CH₂ group.

General Characteristics of members of homologous series are:

1. General Formula — All members of a homologous series share the same general formula. (Same elements and same functional group). For example, the general formula for alkane is C_nH_2n+2, alkene is C_nH_2n, alkyne is C_nH_2n-2.
2. Molecular Mass — Each member of the series differs from the preceding one by the addition of CH₂ group and by molecular mass of 14 a.m.u [CH₂ = 12 + 2 = 14].
3. Physical Properties — The physical properties of the members change gradually showing gradation in properties as the number of carbon atoms per molecule increases, i.e. as molecular mass increases.
   For example, melting point, boiling point and the density of the successive members of the homologous series increase with increase in molecular mass.
4. Chemical Properties — Members of a homologous series have similar chemical properties.
   For example, Methane reacts with chlorine to form methyl chloride:
   CH₄ + Cl₂ ⟶ CH₃Cl + HCl
   Similarly, ethane reacts to form ethyl chloride
   C₂H₆ + Cl₂ ⟶ C₂H₅Cl + HCl
5. Preparation — All the members of a homologous series can be prepared by using the same general method of preparation.