A) Change in chemical properties

Compounds within a homologous series typically have a fixed set of functional groupsthat gives them similar chemical and physical properties. (For example, the series of primary straight-chained alcohols has a hydroxyl at the end of the carbon chain.)

Introduction:
In organic chemistry, a homologous series is a series of compounds with similar chemical properties and the same functional group, but with increasing molecular size and mass. These compounds have a general formula and exhibit a gradation in physical properties. However, there are certain characteristics that are not observed in a homologous series.

Explanation:

a) Change in chemical properties:
In a homologous series, the chemical properties of the compounds remain relatively constant. They have the same functional group, which determines their reactivity and chemical behavior. The presence of the same functional group gives rise to similar chemical reactions and properties. Therefore, change in chemical properties is not observed in a homologous series.

b) Difference in -CH2 and 14u molecular mass:
In a homologous series, the difference in molecular mass between consecutive compounds is usually 14u. This is because each compound in the series differs by a -CH2 group, which has a molecular mass of approximately 14u. The addition of a -CH2 group increases the molecular mass by 14u. Therefore, the difference in molecular mass between consecutive compounds is a characteristic of a homologous series.

c) Gradation in physical properties:
A homologous series exhibits a gradation in physical properties such as boiling point, melting point, density, and solubility. This is because as the molecular size and mass increase, the intermolecular forces between the compounds also increase. These intermolecular forces affect the physical properties of the compounds. Therefore, gradation in physical properties is observed in a homologous series.

d) Same functional group:
A homologous series is characterized by the presence of the same functional group in each compound. The functional group determines the chemical behavior and reactions of the compounds. It imparts similar chemical properties to the compounds in the series. Therefore, the presence of the same functional group is a defining characteristic of a homologous series.

Conclusion:
Of the given options, the characteristic that is not observed in a homologous series is a change in chemical properties. Homologous series exhibit a gradation in physical properties, have the same functional group, and show a difference in molecular mass between consecutive compounds.