(A) is HgS, (B) is HgCl2, (C) is H2S, (D) is HgI2, and (E) is K2HgI4
When black coloured HgS is heated with dil HCl, mercuric chloride (B ) and hydrogen sulphide gas (C) is obtained.
HgS+2HCl→HgCl2+2H2S
Hydrogen sulphide gas turns lead acetate paper black.
Mercury(II) ions react with KI to form orange precipitate of HgI2 which is D. It is soluble in excess of KI to form E which is K2HgI4.
HgCl2+2KI→HgI2↓orangeprecipitate+2KCl
HgI2+2KI(excess)→K2[HgI4]