Out of the following, which reaction gives rise to a colloidal sol:

Options

A. `Cu+HgCI_(2)rarrCuCI_(2)+Hg`
B. `2HNO_(3)+3H_(2)Srarr3S+4H_(2)O+2NO`
C. `2Mg+CO_(2)rarr2MgO+C`
D. `Cu+CuCI_(2)rarrCu_(2)CI_(2)`

Answer

To determine which reaction gives rise to a colloidal sol, we need to analyze the reactions provided and understand the criteria for forming colloids. A colloidal sol is formed when particles are dispersed in a medium, and these particles typically have a size range of 1 nm to 1000 nm.

### Step-by-Step Solution:

1. **Understanding Colloidal Sols**:
- A colloidal sol consists of tiny particles (disperse phase) suspended in a continuous medium (disperse medium). The particle size is crucial; it should be between 1 nm and 1000 nm.

2. **Analyzing the Given Reactions**:
- The question provides a specific reaction:
- **Cu + HgCl2 → CuCl2 + Hg**
- We need to evaluate if this reaction can lead to the formation of a colloidal sol.

3. **Reaction Characteristics**:
- In this reaction, copper (Cu) reacts with mercuric chloride (HgCl2). The products are copper(II) chloride (CuCl2) and mercury (Hg).
- Mercury is a liquid at room temperature, and copper(II) chloride is soluble in water.

4. **Colloidal Formation**:
- For a colloidal sol to form, the product must not be completely soluble in the medium. If the product dissolves completely, it will not form a colloidal system.
- In this case, CuCl2 is soluble in water, which means it will not form a colloidal sol.

5. **Conclusion**:
- Since the reaction results in a soluble product (CuCl2), it does not lead to the formation of a colloidal sol.

### Final Answer:
None of the reactions provided lead to the formation of a colloidal sol.

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