The user wants me to identify a metal ion based on two chemical tests described.

Step 1: Analyze the reaction with Potassium Iodide (KI)

The problem states that the metal ion solution, when treated with KI, gives a red precipitate which dissolves in excess KI to give a colourless solution. Let's examine the reactions of each given metal ion with KI:

• A: Pb²⁺ (Lead(II) ion) $\text{Pb}^{2+} + 2\text{I}^- \rightarrow \text{PbI}_2 \downarrow$ Lead(II) iodide (PbI₂) is a bright yellow precipitate. This does not match the description of a red precipitate. Although PbI₂ dissolves in excess KI to form the colourless complex K₂[PbI₄], the initial precipitate colour is wrong.

• B: Hg²⁺ (Mercury(II) ion) $\text{Hg}^{2+} + 2\text{I}^- \rightarrow \text{HgI}_2 \downarrow$ Mercury(II) iodide (HgI₂) is a scarlet red precipitate. This matches the description. Upon adding excess KI, the precipitate dissolves to form a soluble, colourless complex, tetraiodomercurate(II) ion. $\text{HgI}_2\downarrow + 2\text{I}^-_{\text{(excess)}} \rightarrow [\text{HgI}_4]^{2-}_{\text{(aq)}}$ This reaction sequence perfectly matches the first observation.

• C: Cu²⁺ (Copper(II) ion) $2\text{Cu}^{2+} + 4\text{I}^- \rightarrow 2\text{CuI}\downarrow + \text{I}_2$ This is a redox reaction where Cu²⁺ is reduced to Cu⁺ and I⁻ is oxidized to I₂. The precipitate formed is Copper(I) iodide (CuI), which is white. The solution turns brown due to the liberated iodine. This does not match the description.

• D: Co²⁺ (Cobalt(II) ion) Co²⁺ does not form a precipitate with KI. Cobalt(II) iodide (CoI₂) is soluble in water.

From the first test, we can conclude that the metal ion is likely Hg²⁺.

Step 2: Analyze the reaction with Cobalt(II) Thiocyanate

The problem states that the solution of the metal ion, on treatment with a solution of cobalt(II) thiocyanate (Co(SCN)₂), gives a deep blue crystalline precipitate.

Let's test this reaction with our candidate, Hg²⁺.

• B: Hg²⁺ (Mercury(II) ion) The reaction of Hg²⁺ ions with a solution containing Co²⁺ and thiocyanate (SCN⁻) ions forms cobalt tetrathiocyanatomercurate(II). $\text{Hg}^{2+} + \text{Co}^{2+} + 4\text{SCN}^- \rightarrow \text{Co}[\text{Hg}(\text{SCN})_4] \downarrow$ This compound, Co[Hg(SCN)₄], is a deep blue crystalline precipitate. This reaction is a characteristic confirmatory test for Hg²⁺ ions.

This second observation confirms that the metal ion is indeed Hg²⁺.

Step 3: Conclusion

Both chemical tests described in the question are characteristic reactions of the Hg²⁺ ion.

1. Formation of a scarlet red HgI₂ precipitate, soluble in excess KI.
2. Formation of a deep blue Co[Hg(SCN)₄] precipitate with cobalt(II) thiocyanate.

Therefore, the metal ion is Hg²⁺.