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 see if they produce particles within the colloidal size range (1 nm to 1000 nm).

### Step-by-Step Solution:

1. **Understanding Colloidal Sols**:
- Colloidal sols are mixtures where tiny particles (1 nm to 1000 nm) are dispersed in a medium. This results in a heterogeneous mixture, unlike a homogeneous solution where particles are completely dissolved.

2. **Analyzing the First Reaction**:
- **Reaction**: Cu + HgCl2 → CuCl2 + Hg
- **Analysis**: CuCl2 is soluble in water, forming a homogeneous solution. The particle size is too small to be classified as a colloidal sol.
- **Conclusion**: This reaction does not produce a colloidal sol.

3. **Analyzing the Second Reaction**:
- **Reaction**: HNO3 + H2S → 3 S + 4 H2O + 2 NO
- **Analysis**: The sulfur produced in this reaction can accumulate into larger particles. These particles can range from 1 nm to 1000 nm, fitting the criteria for a colloidal sol.
- **Conclusion**: This reaction produces a colloidal sol.

4. **Analyzing the Third Reaction**:
- **Reaction**: Mg + CO2 → MgO + C
- **Analysis**: Magnesium oxide (MgO) has a high lattice energy and forms large particles, exceeding 1000 nm. Therefore, it does not produce a colloidal sol.
- **Conclusion**: This reaction does not produce a colloidal sol.

5. **Analyzing the Fourth Reaction**:
- **Reaction**: Cu + Cl2 → Cu2Cl2
- **Analysis**: This reaction forms a precipitate, which cannot be easily converted into a colloidal form (peptized). Thus, it does not meet the criteria for a colloidal sol.
- **Conclusion**: This reaction does not produce a colloidal sol.

### Final Answer:
The reaction that gives rise to a colloidal sol is the second one: **HNO3 + H2S → 3 S + 4 H2O + 2 NO**.

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