The physical properties of various members of a homologous series change regularly with an increase in the molecular mass. Variation of some physical properties in a homologous series of hydrocarbons are described below.
(i) Variations in melting and boiling points. Melting and boiling points of hydrocarbons in a homologous series increase with an increase in molecular mass. Thus, a compound containing larger number of carbon atoms will have higher melting and boiling points.
(ii) Variation in physical state: Hydrocarbons having lesser number of carbon atoms have lower melting and boiling points. As a result , under normal conditions,
(a) Hydrocarbons containing lesser number of carbon atoms are gases.
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(b) Hydrocarbons containing large number of carbon atoms are solids.
(c) Hydrocarbons containing intermediate number of carbon atoms are liquids.
For example, Hydrocarbons containing 1-4 carbon atoms are gases, those containing 5-13 carbon atoms are liquids, and those containing more than 14 carbon atoms are solids.