(A) Mercuric ions reacts with iodide ions to form red ppt of mercuric iodide.
Hg2+(aq)+I−(aq)⟶HgI2↓(Red ppt.)
(B) Cupric ions reacts with ferrocyanide ion to form brown ppt of cupric ferrocyanide.
2Cu2+(aq)+[Fe(CN)6]4−(aq)⟶Cu2[Fe(CN)6]↓(Brown ppt.)
(C) Magnesium ions react with hydrogen phosphate ions in presence of ammonia to form white ppt of magnesium ammonium phosphate.
Mg2+(aq)+NH3(aq)+HPO2−4(aq)⟶MgNH4PO4↓(White)
(D) Lead (II) ions reacts with chromate ions to form yellow ppt of lead chromate.
Pb2+(aq)+CrO2−4(aq)⟶PbCrO4↓(Yellow)