CBSE Class 10 Science 2018 · Paper 31(B) · Question 7

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. Characteristics (any two): 1. All members are represented by the same general formula. 2. Successive members differ by a $-{\text{CH}}_2-$ unit, that is by $14$ u in molecular mass. 3. All members have similar chemical properties. 4. Physical properties such as melting and boiling points change gradually as the molecular mass increases. Compounds: ${\text{C}}_2{\text{H}}_2$ and ${\text{C}}_3{\text{H}}_4$, because both are alkynes with the general formula ${\text{C}}_n{\text{H}}_{2n-2}$ ($n=2$ and $n=3$), and they differ by one ${\text{CH}}_2$ unit ($26$ u and $40$ u).