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A student has been collecting silver coi...

A student has been collecting silver coins and copper coins. One day she observed a black coating on silver coins and a green coating on copper coins. Which chemical phenomenon is responsible for these coatings ? Write the chemical names of black and green coatings ?

Text Solution

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The phenomenon is known as corrosion. Air contains traces of hydrogen sulphide gas which reacts with silver metal present in the coin to form silver sulphide. It is black in colour.
`2Ag + underset(("Air"))(H_(2)S) rarr underset(("Black"))underset("Silver sulphide")(Ag_(2)S)+H_(2)`
Similarly, copper present in the coin reacts with oxygen and traces of both carbon dioxide and water vapours present in air to form a green mass. It is chemically basic copper carbonate :
`2Cu + underset(("Present in air")) ubrace (H_(2)O + CO_(2)) + O_(2) rarr underset(("Green"))underset("Basic copper carbonate")(Cu(OH)_(2).CuCO_(3))`
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