Which of the following is not correct?

\(NH_4Cl  +  \text{Nesseler's reagent}  \rightarrow  \text{Blue colour}\)

To determine which of the given reactions is not correct, let's evaluate each one:

(1) \(FeCl_3 + KCNS \rightarrow \text{Red colour}\)

This reaction is correct. When potassium thiocyanate (KCNS) is added to ferric chloride (FeCl₃) solution, it forms a complex called ferric thiocyanate, which is a deep red color.

(2) \(Ki + PbCl_2 \rightarrow \text{Yellow ppt}\)

This reaction is correct. When potassium iodide (KI) is added to lead chloride (PbCl₂) solution, it forms lead iodide, which is yellow and insoluble in water, resulting in a yellow precipitate (ppt).

(3) \(NH_4Cl + \text{Nesseler's reagent} \rightarrow \text{Blue colour}\)

This reaction is not correct. Nesseler's reagent is a solution of potassium tetraiodomercurate (K₂[HgI₄]). When ammonia gas (NH₃) reacts with Nesseler's reagent, it forms a brown color complex, not a blue color. The correct reaction would be:

\(NH_3 + \text{Nesseler's reagent} \rightarrow \text{Brown colour}\)

(4) \(DMG + Ni^{2+} + NH_4OH \rightarrow \text{Scarlet red ppt}\)

This reaction is correct. DMG (dimethylglyoxime) is a ligand that reacts with nickel(II) ions (Ni²⁺) in the presence of ammonia (NH₄OH) to form a scarlet red precipitate. This is a common test for the presence of nickel ions.

So, the incorrect reaction is: (3) \(NH_4Cl + \text{Nesseler's reagent} \rightarrow \text{Blue colour}\)