Question

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 name of black and green coatings.

Answer 1

The phenomena responsible for the formation of a coating on copper and silver objects is known as corrosion. Silver turns black due to the formation of silver sulphide$\left({\mathrm{Ag}}_2\mathrm{S}\right)$ while copper turns green due to the formation of a mixture of copper carbonate and copper hydroxide$\left[\right({\mathrm{CuCO}}_3).(\mathrm{Cu}\left(\mathrm{OH}{)}_2)\right]$.
$2\mathrm{Cu}+{\mathrm{CO}}_2+{\mathrm{H}}_2\mathrm{O}\to \left[\right({\mathrm{CuCO}}_3).(\mathrm{Cu}\left(\mathrm{OH}{)}_2\right)]$
$2\mathrm{Ag}+{\mathrm{H}}_2\mathrm{S}\to {\mathrm{Ag}}_2\mathrm{S}+{\mathrm{H}}_2$