CBSE Class 10 Science 2019 · Paper 31/2/1 · Question 9

A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain, so that successive members differ by a ${\text{CH}}_2$ unit. Example: the alcohols, ${\text{CH}}_3\text{OH}$, ${\text{C}}_2{\text{H}}_5\text{OH}$, ${\text{C}}_3{\text{H}}_7\text{OH}$, ${\text{C}}_4{\text{H}}_9\text{OH}$ (the alkanes ${\text{CH}}_4$, ${\text{C}}_2{\text{H}}_6$, ${\text{C}}_3{\text{H}}_8$ are another). Characteristics: (i) All members have the same general formula (${\text{C}}_n{\text{H}}_{2n+1}\text{OH}$ for the alcohols). (ii) All members have the same functional group ($-\text{OH}$ for the alcohols). (iii) Successive members differ by a ${\text{CH}}_2$ unit, that is, by $14$ u in molecular mass. They also show similar chemical properties, and their physical properties change gradually; either of these is accepted as a characteristic too.