Question

Samuel had a silver coin which turned black. He kept the coin in a bowl lined with aluminium foil. Then he filled the bowl with water and boiled it. After some time, he found that the coin has become new. Its blackness disappeared. How did it happen?

Answer 1

The blackness of silver coin is due to the formation of silver sulphide on its surface due to its exposure to air. On boiling, the aluminium foil reacts with the layer of silver sulphide and displaces silver from silver sulphide to form aluminium sulphide and silver. This makes the coin shiny.
3Ag2S + 2Al -----> 6Ag + Al₂S₃