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The series which consists of similar structures and similar chemical properties in which the successive compounds differs by a CH₂ group is termed as homologous series.

The two characteristics of homologous series are as follows:

1. The members of the homologous series consists of same general formula such as Alkanes have general formula C_nH_2n+2

2. The two adjacent homologues have a difference of molecular mass of 14 u.

The molecular formula of two consecutive members of homologous series of aldehydes are :

1. Methanol: H-CHO

2. Ethanal: CH₃–CHO

3. Propanal: C₂H₅ – CHO

(i) The physical properties of the compounds is determined by the carbon chain attached to the aldehyde group.

(ii) The chemical properties of the compound is determined by the functional group attached to the aldehyde group.