When `FeCl_3` solution is added to NaOH a negatively charged sol is obtained. It is due to the:

Options

A. Presence of basic group
B. Preferential adsorption of `OH^(-)` ions
C. Self dissociation
D. Electron capture by sol particles

Answer

To solve the question of why a negatively charged sol is obtained when FeCl₃ solution is added to NaOH, we can break down the process into a series of steps:

### Step-by-Step Solution:

1. **Identify the Reactants**:
- We start with two reactants: **FeCl₃ (Ferric chloride)** and **NaOH (Sodium hydroxide)**.

2. **Reaction Between FeCl₃ and NaOH**:
- When FeCl₃ is added to NaOH, a reaction occurs where ferric hydroxide (Fe(OH)₃) precipitate is formed along with sodium chloride (NaCl).
- The balanced reaction can be represented as:
\[ FeCl_3 + 3NaOH \rightarrow Fe(OH)_3 \downarrow + 3NaCl \]

3. **Formation of Ferric Hydroxide**:
- Ferric hydroxide (Fe(OH)₃) precipitate is formed. The size of this precipitate is larger than colloidal particles, typically greater than 1000 nanometers.

4. **Excess NaOH**:
- If an excess of NaOH is added, it introduces more hydroxide ions (OH⁻) into the solution.

5. **Adsorption of Hydroxide Ions**:
- The hydroxide ions (OH⁻) from the excess NaOH preferentially adsorb onto the surface of the ferric hydroxide precipitate. This process is known as **preferential adsorption**.
- The adsorption of OH⁻ ions creates a double layer on the surface of the ferric hydroxide particles.

6. **Formation of Smaller Particles**:
- Due to the repulsion between the negatively charged OH⁻ ions on the surface, the larger ferric hydroxide particles dissociate into smaller particles. These smaller particles fall within the colloidal size range (1 to 1000 nanometers).

7. **Charge of the Colloidal Particles**:
- The smaller particles formed are negatively charged due to the adsorption of OH⁻ ions. This results in the formation of a negatively charged sol.

8. **Conclusion**:
- Therefore, the negatively charged sol is obtained due to the **preferential adsorption of OH⁻ ions** on the surface of the ferric hydroxide particles.

### Final Answer:
The answer to the question is **option 2: preferential adsorption of OH⁻ ions**.

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