A solution of a metal ion when treated with `KI` gives a red precipitate which dissolves in excess `KI` to give a colourless solution.Moreover, the solution of metal ion on treatment with a solution of cobalt `(II)` thiocyanate gives rise to a deep blue crystalline precipitate.The metal ion is:

Options

A. `Pb^(2+)`
B. `Hg^(2+)`
C. `Cu^(2+)`
D. `Co^(2+)`

Explanation

To solve the problem, we need to identify the metal ion based on the reactions described. Let's break it down step by step.

### Step 1: Analyze the Reaction with KI
When a solution of the metal ion is treated with potassium iodide (KI), it produces a red precipitate. This indicates that the metal ion likely forms a compound with iodine.

**Hint:** Look for metal ions that form colored precipitates with iodide ions.

### Step 2: Identify the Red Precipitate
The red precipitate formed is likely to be mercuric iodide (HgI2). The reaction can be represented as:
\[ \text{Hg}^{2+} + 2 \text{KI} \rightarrow \text{HgI}_2 \downarrow + 2 \text{K}^+ \]

**Hint:** Consider common metal ions that form red precipitates with iodide.

### Step 3: Examine the Effect of Excess KI
When excess KI is added, the red precipitate dissolves to form a colorless solution. This indicates the formation of a soluble complex. The reaction can be represented as:
\[ \text{HgI}_2 + 2 \text{KI} \rightarrow \text{K}_2\text{HgI}_4 \]

**Hint:** Look for metal ions that can form soluble complexes with iodide ions.

### Step 4: Analyze the Reaction with Cobalt(II) Thiocyanate
The problem states that the solution of the metal ion, when treated with cobalt(II) thiocyanate (Co(SCN)2), produces a deep blue crystalline precipitate. This suggests the formation of a complex between the metal ion and cobalt thiocyanate.

**Hint:** Consider the known complexes that form between cobalt thiocyanate and metal ions.

### Step 5: Identify the Deep Blue Crystalline Precipitate
The deep blue crystalline precipitate formed is likely to be a complex of mercuric ion with cobalt thiocyanate, represented as:
\[ \text{Hg}^{2+} + \text{Co(SCN)}_2 \rightarrow \text{CoHg(SCN)}_4 \downarrow \]

**Hint:** Recall that certain metal ions form characteristic colored complexes with thiocyanate.

### Conclusion
Based on the reactions described, the metal ion that fits all the criteria is **mercury (II) ion, Hg²⁺**.

### Final Answer
The metal ion is: **Hg²⁺**.