Homologous series

In chemistry, a homologous series is a series of chemical compounds with the same general formula. Usually these compounds vary by a single parameter such as the length of a carbon chain. Examples of such series are the straight-chained alkanes (paraffins), and some of their derivatives (such as the primary alcohols, aldehydes, and (mono)carboxylic acids). 
Compounds within a homologous series typically have a fixed set of functional groups that gives them similar chemical and physical properties. For example, the series of primary straight-chained alcohols has an hydroxyl at the end of the carbon chain. These properties typically change gradually along the series, and the changes can often be explained by small differences in molecular size and mass.