As H2S is added to the green solution, black CuS forms in a colorless solution of Zn2+(aq).
When more H2S is added, a second precipitate of white ZnS forms.
Selective Precipitation of Ions
Ions can be separated from each other based on their salt solubilities.
Example: if HCl is added to a solution containing Ag+ and Cu2+, the silver precipitates (Ksp for AgCl is 1.8 ? 10-10) while the Cu2+ remains in solution.
Removal of one metal ion from a solution is called selective precipitation.
