A homologous series is a group of organic compounds that have the same general formula and the same functional group, and in which the successive compounds differ by a CH2 unit. The members of a homologous series are called homologues. For example, the alkane series is a homologous series with the general formula CnH2n+2, where n is the number of carbon atoms. The first few members of the alkane series are methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), and so on.

Some characteristics of homologous series are:

• They have the same general formula.
• They have similar chemical properties.
• They have a gradual increase in physical properties such as boiling point, melting point, and density as the number of carbon atoms increases.

An example of a homologous series is the carboxylic acid series, which has the general formula CnH2n+1COOH. The first few members of this series are formic acid (HCOOH), acetic acid (CH3COOH), propionic acid (C2H5COOH), butyric acid (C3H7COOH), and so on.