Methane, ethane, propane and butane are all members of the alkane homologous series. Explain why these four substances belong to the same homologous series and describe how their physical properties change as the number of carbon atoms increases.

All alkanes share the general formula CnH2n+2, so methane, ethane, propane and butane all satisfy this rule. Each successive member differs from the previous one by a single CH2 unit, giving a clear homologous relationship. Because the carbon–hydrogen skeleton is the same, all alkanes show very similar chemical behaviour – they react in the same way and undergo the same types of reactions.

As the carbon chain length increases, the molecules become larger and heavier. This leads to higher boiling points – methane boils at –161.5 °C, ethane at –88.6 °C, propane at –42.1 °C and butane at 0.5 °C. Viscosity also rises with chain length, so the liquids become thicker and less runny as the number of carbon atoms increases.