Why does silver turn black or green when it is old? Or, why does a silver coin blacken when exposed to sulphuretted hydrogen?
Answer: Air contains small amounts of H2S gas. This H2S gas reacts with silver to produce black-coloured silver sulphide. As a result, gradually, i.e. when old, silver sulphide deposits on shiny silver objects, turning the object black.
2Ag + H2S = Ag2S + H2