Which is not true for a detergent molecule ?

Options

A. It has a non-polar organic part and a polar group .
B. It is not easily biodegraded .
C. It is a sodium salt of fatty acids.
D. It is a surface active reagent .

Explanation

To determine which statement about detergent molecules is not true, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Detergent Structure**:
- Detergent molecules typically consist of two parts: a non-polar organic part (hydrophobic tail) and a polar group (hydrophilic head).
- **Statement 1**: "It has a non-polar organic part and a polar group."
- **Evaluation**: This statement is true because all detergents have this dual structure.

2. **Biodegradability**:
- Many synthetic detergents are not easily biodegradable, which can lead to environmental concerns.
- **Statement 2**: "It is not easily biodegradable."
- **Evaluation**: This statement is true for many detergents, especially synthetic ones.

3. **Chemical Composition**:
- Detergents are often sodium salts of sulfonic acids, not fatty acids.
- **Statement 3**: "It is a sodium salt of fatty acids."
- **Evaluation**: This statement is not true. Detergents are typically sodium salts of sulfonic acids, making this statement incorrect.

4. **Surface Activity**:
- Detergents are known as surface-active agents because they reduce the surface tension of water, allowing it to spread and penetrate fabrics more easily.
- **Statement 4**: "It is a surface active reagent."
- **Evaluation**: This statement is true as detergents function as surface-active agents.

### Conclusion:
The incorrect statement about detergent molecules is **Statement 3**: "It is a sodium salt of fatty acids." Therefore, the answer is **C**.