Answer. The group of organic compounds consisting of same functional group, having same general formula, having similar chemical properties and gradually changing physical properties is called homologous series. Successive members of the series differ by one unit of –CH2-. For example, alkenes consist of functional group -C=C-, their general formula is CnH2n. All the members of the series have similar chemical properties. Their physical properties such as molecular masses increase gradually. Each member of the series differ from its lower or higher member by one unit –CH2― e.g. Ethene (C2H4), Propene (C3H6), Butene (C4H8) and so on.