There are a range of answers you could give: Homologous series' are chemically similar or chemically the same or react in the same way. They contain the same functional group, the same general formula (e.g. CnH2n for alkenes) and they differ by CH2. There is also a gradual change in physical properties as the relative molecular mass increases. For example, the boiling points of alkanes increase regularly with the increase in the number of carbon atoms. As an example, CH4 has a boiling point of 109K whereas C6H14 has a boiling point of 342K.