A red solid is insoluble in water. However, it becomes soluble if some KI is added to water. Heating rod solid in a test tube produces violet coloured fumes and droplets of metal appear on the cooler parts of test tube. The red solid is

Options

A. `Pb_(3)O_(4)`
B. `HgI_(2)`
C. `HgO`
D. `(NH_(4))_(2)Cr_(2)O_(7)`

Answer

To solve the question regarding the red solid that is insoluble in water but becomes soluble with the addition of KI, and produces violet fumes upon heating, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Characteristics of the Red Solid**:
- The solid is described as red and insoluble in water.
- It becomes soluble when KI (potassium iodide) is added to water.
- Upon heating, it produces violet-colored fumes and droplets of metal.

2. **Consider Possible Compounds**:
- The description of a red solid suggests the possibility of mercury(I) iodide (HgI2), which is known to be a scarlet red precipitate.
- Other compounds may also be red, but we need to focus on those that fit all the criteria.

3. **Solubility in KI**:
- HgI2 is insoluble in water but is known to form a complex with KI, resulting in the formation of K2HgI4·HgI2, which is soluble in water. This matches the property of becoming soluble when KI is added.

4. **Heating the Red Solid**:
- When HgI2 is heated, it decomposes to produce mercury (Hg) droplets and iodine (I2) vapors. The iodine vapor is recognized for its violet color, which aligns with the information given in the question.

5. **Conclusion**:
- Based on the properties described, the red solid in question is confirmed to be mercury(I) iodide (HgI2). It satisfies all the conditions: it is red, insoluble in water, soluble in KI, and produces violet fumes upon heating.

### Final Answer:
The red solid is **HgI2 (mercury(I) iodide)**.