Which of the following is true about the homologous series?

Correct option is A

The correct answer is (a) Each successive member differs by -CH2-group.

Explanation:

• A homologous series is a family of organic compounds possessing similar chemical properties and a common general formula.
• The defining hallmark of any homologous series is that each successive member logically differs from the previous one by a standard -CH_2-unit (a methylene group).
• Because they differ by a -CH_2- group, their molecular masses systematically differ by exactly 14 atomic mass units (u).
• For example, in the alkane series: Methane is CH_4, and the next member Ethane is C_2H_6, establishing a clear difference of CH_2.

Information Booster:

• All constituent members of a particular homologous series are effectively represented by the same general chemical formula (e.g., $C_nH_{2n+2}$ for alkanes).
• As molecular mass progressively increases within a homologous series, physical properties systematically undergo a gradual change (gradation).
• For instance, melting points and boiling points predictably increase with ascending molecular mass.

Additional Knowledge:
All members have different functional groups. (Option b)

• This is fundamentally incorrect. The chemical similarity of a homologous series occurs specifically because all members inherently share the same functional group.

All members have the same molecular weight. (Option c)

• Because each member formally adds a $-CH_2-$ group, molecular weights cannot remain the same; they constantly increase by 14 u.

All members have the same boiling point. (Option d)

• Physical properties precisely depend on molecular mass and intermolecular forces, which elevate as the chain lengthens. Thus, boiling points steadily increase, rather than remaining the same.

So the correct answer is (a)