A homologous series is a series of organic compounds which belong to the same family i.e., possess same functional group) and show similar chemical properties. The members of this series are called homologous and differ from each other by the number of `CH_2`, units in the main carbon chain.
Which of the following is not the property of a homologous series ?

Options

A. They show similar chemical properties.
B. They differ by 14 units by mass.
C. They all contain double bond
D. They can be represented by a general formula

Answer

To determine which of the following is not a property of a homologous series, we need to analyze the properties of homologous series in organic chemistry.

### Step-by-Step Solution:

1. **Definition of Homologous Series**: A homologous series is a group of organic compounds that share the same functional group and exhibit similar chemical properties. Each successive member of the series differs by a `CH₂` unit.

2. **Properties of Homologous Series**:
- **Same Functional Group**: All members of a homologous series have the same functional group, which means they will undergo similar chemical reactions.
- **Similar Chemical Properties**: Due to the presence of the same functional group, the members of the series will display similar chemical properties.
- **General Formula**: Members of a homologous series can be represented by a general formula. For example, alkanes follow the general formula CₙH₂ₙ₊₂.
- **Difference by CH₂ Unit**: Each successive member differs from the previous one by a `CH₂` unit, which also corresponds to an increase of 14 amu in molar mass.

3. **Identifying the Incorrect Property**:
- The question asks which of the provided options is NOT a property of a homologous series.
- One of the options states that "they all contain double bonds." This is incorrect because not all members of a homologous series have double bonds; for instance, alkanes (a type of homologous series) only contain single bonds.

4. **Conclusion**: The property that is NOT true for a homologous series is that "they all contain double bonds."

### Final Answer:
The property that is NOT a characteristic of a homologous series is: **"they all contain double bonds."**

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