A gold sol is prepared by :

Options

A. dissolving gold in Aqua-regia and precipitation by dilution
B. double decomposition of `AuCl_(3)`
C. Bredy's arc method
D. grinding in a colloidal mill

Answer

To prepare a gold sol, we can follow these steps based on the Brady's arc method:

### Step-by-Step Solution:

1. **Understanding Gold Sol**:
- A gold sol is a colloidal suspension of gold nanoparticles in a fluid, typically water. It is important to understand what a gold sol is before we discuss its preparation.

2. **Identifying the Preparation Method**:
- Among the options provided, we need to identify the correct method for preparing gold sol. The options are:
a) Dissolving gold in aqua regia and precipitation by dilution
b) Double decomposition of AuCl3
c) Brady's arc method
d) Grinding in a colloidal mill

3. **Selecting the Correct Option**:
- The correct method for preparing gold sol is the **Brady's arc method**. This method is specifically used for preparing colloidal solutions of metals such as gold, platinum, and silver.

4. **Understanding the Brady's Arc Method**:
- The Brady's arc method involves electrical disintegration. In this process, an electrical arc is struck between two electrodes made of the metal (gold in this case).
- These electrodes are immersed in a dispersion medium (like water).

5. **Process Description**:
- The intense heat generated by the electrical arc vaporizes the metal.
- The vaporized metal then condenses to form particles of colloidal size.
- These colloidal particles acquire charges from the surrounding solution, which helps in stabilizing them.

6. **Conclusion**:
- Therefore, the gold sol is prepared by the **Brady's arc method**, which is option C.

### Final Answer:
The gold sol is prepared by **option C: Brady's arc method**.

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