CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 9(b)

A homologous series is a series of carbon compounds in which the same functional group substitutes for hydrogen in a carbon chain. Successive members differ by a $-{\text{CH}}_2-$ unit ($14$ u), have similar chemical properties and show a gradual change in physical properties, and the series can be represented by one general formula. (i) Alcohols: ${\text{C}}_n{\text{H}}_{2n+1}\text{OH}$ ($n=1,2,3,\dots$). (ii) Aldehydes: ${\text{C}}_n{\text{H}}_{2n+1}\text{CHO}$ ($n=0,1,2,\dots$), which is the same as ${\text{C}}_m{\text{H}}_{2m}\text{O}$ ($m=1,2,3,\dots$) when every carbon atom is counted. First members (structures drawn in the figure): methanol, ${\text{CH}}_3\text{OH}$, in which one carbon atom is bonded to three hydrogen atoms and an $-\text{OH}$ group; and methanal (formaldehyde), $\text{HCHO}$, in which one carbon atom is bonded to two hydrogen atoms and double-bonded to an oxygen atom.