Which of the following reactions leads to the formation of a substance in the colloidal state?

Options

A. `Cu+HgCl_2 to CuCl_2+Hg`
B. `2HNO_3+3H_2S to 3S+4H_2O+2NO`
C. `2Mg+CO_2 to 2MgO+C`
D. `Cu+CuCl_2 to Cu_2Cl_2`

Answer

To determine which of the given reactions leads to the formation of a substance in the colloidal state, we will analyze each option step by step.

### Step 1: Understand the definition of colloidal state
A colloidal state refers to a mixture where one substance is dispersed evenly throughout another, with particle sizes typically ranging from 1 nanometer to 1 micrometer. Colloids can be in various forms such as sols, gels, emulsions, and aerosols.

### Step 2: Analyze the first reaction
**Reaction 1:** Copper + HgCl2 → Copper chloride + Hg

- In this reaction, copper reacts with mercury(II) chloride to form copper chloride and mercury.
- Copper chloride is an ionic compound and does not exhibit properties of a colloid.
- Mercury is a metal and does not form a colloidal solution in this context.

**Conclusion:** This reaction does not lead to the formation of a colloidal substance.

### Step 3: Analyze the second reaction
**Reaction 2:** HNO3 + H2S → Sulfur + H2O + 2NO

- Here, hydrogen sulfide (H2S) reacts with nitric acid (HNO3) to produce sulfur, water, and nitrogen oxides.
- The sulfur produced can aggregate into larger particles that fall within the colloidal size range.
- This means that sulfur can exist as a colloidal solution, known as sulfur sol.

**Conclusion:** This reaction leads to the formation of a substance in the colloidal state.

### Step 4: Analyze the third reaction
**Reaction 3:** (Assume a hypothetical reaction for analysis)

- Without specific details, we can assume that if the reaction produces ionic compounds or does not involve aggregation of particles into the colloidal range, it will not form a colloidal substance.

**Conclusion:** If the products are ionic or do not aggregate, this reaction does not lead to a colloidal state.

### Step 5: Analyze the fourth reaction
**Reaction 4:** (Assume another hypothetical reaction for analysis)

- Similar to the third reaction, if the products are not capable of forming aggregates in the colloidal size range, then this reaction will not lead to a colloidal state.

**Conclusion:** If the products are ionic or do not aggregate, this reaction does not lead to a colloidal state.

### Final Conclusion
From the analysis, only the second reaction (HNO3 + H2S → Sulfur + H2O + 2NO) leads to the formation of a substance in the colloidal state. Therefore, the correct answer is **Option 2**.

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