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.