Chemistry - Salt Analysis Question with Solution | TestHub

Step 1 & 4 (Separation): Excess NaOH precipitates all four ions (Cu²⁺, Hg²⁺, Co²⁺, Ni²⁺), meaning Option D is correct. After step 3, CuS and HgS form a black precipitate; adding $2MHN{O}_3$ dissolves only CuS into a colored solution while HgS stays insoluble, so Option A is correct.

Step 7 (Mercury Confirmation): HgS is dissolved in aqua regia (Step 6) to form HgCl₂. Adding SnCl₂ reduces it first to white Hg₂Cl₂ and then to a black metallic precipitate of liquid mercury (Hg), making Option B correct.

Step 8 (Cobalt Confirmation): Reacting Co²⁺ with NH₄SCN and HgCl₂ yields a deep blue precipitate of Co[Hg(SCN)₄], not dark red, which makes Option C incorrect.