Video Transcript
Adding AgNO3 to a Na2S solution forms Ag2S. What is the color of the precipitate resulting from the formation of Ag2S? (A) White, (B) yellow, (C) brown, or (D) black.
Adding silver nitrate to a solution can be used as a confirmatory test to determine if sulfide ions are present. The primary test for sulfide ions in an unknown salt is conducted by first adding a dilute acid, such as hydrochloric acid. The reaction between the sulfide anions in the salt and the hydrogen cations from the acid produces the foul-smelling gas hydrogen sulfide. The presence of hydrogen sulfide gas is detected using a paper moistened with lead(II) acetate. The presence of hydrogen sulfide and, thus, sulfide ions in the salt is confirmed when a black precipitate is formed on the paper. This black precipitate is lead(II) sulfide, a product in the reaction between hydrogen sulfide and lead(II) acetate.
Then, a confirmatory test is generally performed to further verify the results from the primary test. In the confirmatory test described by the question, silver nitrate is added to a solution of the unknown salt. If sulfide ions are present in the solution, as they would be in a solution of sodium sulfide, they will react with the silver ions to produce a black precipitate. This black precipitate is silver sulfide. Therefore, the color of a precipitate resulting from the formation of Ag2S is answer choice (D) black.
