The observation that a precipitate forms when an aqueous solution suspected of containing Ba2+ or Pb2+ or both is mixed with sulfuric acid suggests that both Ba2+ and Pb2+ ions are present in the solution. When sulfide ions are added to the original solution and no precipitate forms, it indicates the absence of Pb2+ ions.
To understand this, let's analyze the chemical reactions that occur in each test:
1. When an aqueous solution suspected of containing Ba2+ or Pb2+ or both is mixed with sulfuric acid, a precipitation reaction occurs. Sulfuric acid (H2SO4) reacts with Barium ions (Ba2+) and Lead ions (Pb2+) to form insoluble precipitates: Barium sulfate (BaSO4) and Lead sulfate (PbSO4):
Ba2+ (aq) + H2SO4 (aq) → BaSO4 (s) + 2H+ (aq)
Pb2+ (aq) + H2SO4 (aq) → PbSO4 (s) + 2H+ (aq)
The formation of a precipitate indicates the presence of Ba2+ and/or Pb2+ ions in the original solution.
2. In the second test, when the original solution is mixed with an aqueous solution of sodium sulfide (Na2S), no precipitate forms. This suggests that there is no presence of Pb2+ ions.
The reaction between Sodium sulfide (Na2S) and Barium ions (Ba2+) forms an insoluble precipitate of Barium sulfide (BaS):
Ba2+ (aq) + S2- (aq) → BaS (s)
Since no precipitate forms, it indicates the absence of Pb2+ ions in the original solution.
To summarize, the first test, where a precipitate forms upon mixing the original solution with sulfuric acid, indicates the presence of Ba2+ and/or Pb2+ ions. The second test, where no precipitate forms upon mixing the original solution with sodium sulfide, suggests the absence of Pb2+ ions, but the presence of Ba2+ ions in the original solution.