Original: Since bismuth ( III ) iodide is insoluble in water , an aqueous solution can be tested for the presence of Bi3 + ions by adding a source of iodide such as potassium iodide . A black precipitate of bismuth ( III ) iodide indicates a positive test .
Simplified: Bismuth ions form a dark gray precipitate when reacted with potassium iodide because bismuth ( III ) iodide was made .