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.


(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.