Ans. Homologous series.
A group of organic compounds containing same functional group which can be represented by the same general formula and which have similar trends in their properties are called a homologous series. In the general formula of compounds of a homologous series, the symbol R is used to represent its functional group. Alcohols (R – OH), Aldehydes (R – CHO), Carboxylic acid (R – COOH), Ketones(R-C(=O)-R') are some of homologous series and their general formulae. The alkanes family is a homologous family which have linear molecules. Examples: Ethane C2H6, Propane C3H8, Butane C4H10, Pentane C5H12.
Four characteristics of a homologous series are:
(i) The general formula of all compounds in the series is the same.
(ii) They have the same functional group.
(iii) Their physical properties such as melting point, boiling point, density, generally show a gradual change with increase of molecular formula in the series.
(iv) Their chemical properties show close similarities due to the pressure of the same functional group in them.
(v) Consecutive members of the series differ from each other by -CH2- group which is known as the methylene group and their molecular weight differs by 14 units.