Colloidal solution of gold cannot be prepared by

Options

A. Bredig's arc method
B. Mechanical dispersion
C. Reduction of gold chloride
D. Exchange of solvents

Explanation

To determine which method cannot be used to prepare a colloidal solution of gold, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Colloidal Solutions**:
- A colloidal solution consists of tiny particles dispersed throughout a medium. In the case of gold, we are looking for methods to create a stable dispersion of gold particles.

2. **Analyzing the Methods**:
- **Bridge Arc Method**: This method involves creating an arc between two electrodes in the presence of a liquid. When gold is vaporized, it can condense into colloidal particles. This method can produce a colloidal solution of gold.

- **Can Method**: This method typically refers to the use of a container to facilitate the dispersion of particles. It can also be used to produce colloidal solutions, including that of gold.

- **Dispersion Method**: This involves breaking down bulk gold into smaller particles, which can then be dispersed in a medium. This is a common method to prepare colloidal gold.

- **Reduction of Gold Chloride**: This method involves reducing gold chloride (AuCl₃) to form colloidal gold. This is a well-known method for producing colloidal gold solutions.

- **Exchange of Solvent**: This method refers to replacing one solvent with another. In the context of gold, most of its salts are soluble in water, and simply changing the solvent does not lead to the formation of a colloidal solution. This method does not facilitate the formation of colloidal gold.

3. **Conclusion**:
- After analyzing all the methods, it is clear that the **exchange of solvent** is the method that cannot be used to prepare a colloidal solution of gold. Therefore, the correct answer is option **fourth**: exchange of solvent.

### Final Answer:
The colloidal solution of gold cannot be prepared by **exchange of solvent**.

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