(c) Ammonia gas reacts with hydrogen chloride gas to form dense white fumes of ammonium chloride. This is used as a test to detect the presence of ammonia gas as per the reaction. NH3(g)+HCl(g)⟶NH4Cl(s) Nessler's reagent is a solution containing K2HgI4 and KOH. lodide and mercury ions react with ammonia under alkaline conditions to produce a reddish brown complex, iodide of million's base (H2N−Hg−O−Hg−I) formed.