Cadmium sulfide precipitate. Cadmium sulfide (CdS) precipitate (bright yellow-orange) formed by adding 0.2 M solution of sodium sulfide (Na2S) to 0.2 M solution of cadmium sulfate (CdSO4). The reaction is Na2S + CdSO4 -> CdS + Na2SO4. This is an example of a double replacement reaction. This is a classic demonstration of a chemical reaction where two clear solutions, when mixed, produce a brightly colored precipitate.