Which method of preparation of sol involves
chemical reaction?

Options

A. Hydrolysis
B. Mechanical dispersion
C. Exchange of solvent
D. All of these

Answer

To determine which method of preparation of a sol involves a chemical reaction, we will analyze the given methods: hydrolysis, mechanical dispersion, and exchange of solvent.

### Step-by-Step Solution:

1. **Understanding Hydrolysis**:
- Hydrolysis is a chemical reaction where a salt is reacted with water to form a colloidal solution.
- For example, when ferric chloride (FeCl3) is boiled with water, it undergoes hydrolysis to form ferric hydroxide (Fe(OH)3) and hydrochloric acid (HCl).
- The reaction can be represented as:
\[ \text{FeCl}_3 + 3\text{H}_2\text{O} \rightarrow \text{Fe(OH)}_3 + 3\text{HCl} \]
- Since this involves a chemical change, hydrolysis is a method that involves a chemical reaction.

2. **Understanding Mechanical Dispersion**:
- In mechanical dispersion, a substance is finely ground and mixed with a dispersion medium.
- This method does not involve any chemical reaction; it is purely a physical process of breaking down a substance into smaller particles and dispersing them.
- Therefore, mechanical dispersion does not involve a chemical reaction.

3. **Understanding Exchange of Solvent**:
- The exchange of solvent method involves taking a solution of a substance in one solvent and pouring it into another solvent where the substance is less soluble.
- This process also does not involve any chemical reaction; it is a physical change where the solubility of the substance changes due to the different solvents.
- Hence, the exchange of solvent does not involve a chemical reaction.

4. **Conclusion**:
- Among the methods analyzed, only hydrolysis involves a chemical reaction.
- Therefore, the correct answer to the question is **A) Hydrolysis**.