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 sometime, he found that the coin has become new. Its blackness disappeared. How did it happen?
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When a silver coin is placed in a bowl lined with aluminium foil and filled with water and boiled, the coin appears to be new for the removal of the sulphur that had formed silver sulphide and tarnished the silver coin.
Explanation:
- Silver has a tendency to become black when exposed to air for a long time.
- This is because of hydrogen sulfide, a compound that is present in the air.
- When silver comes into contact with H2S, a chemical reaction takes place and a black layer is formed of Silver sulphide.
- When such a black silver coin is placed in a bowl lined with aluminium foil and boiled with water a reverse reaction takes place.
- Aluminium reacts with the sulphide ion to form aluminium sulphide.
- The silver now regains its original colour as it returns to its elemental state .
- Hence the silver coin again appears to be new and its blackness completely disappears.
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Explanation:
The blackness of silver coin is due to the formation of silver sulfide in its surface due to its exposure to air. On boiling, the aluminium foil reacts with the layer of silver sulfide and displaces silver from silver sulfide to from aluminium sulphide and silver. This makes the coin shiny.
3Ag2s +2Al = 6Ag + Al2s3
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