Answer :

when metals are exposed to air to a very long period of time then they loses their shininess and appears dull. That is because of the formation of oxide, carbonate or sulphides thin layer on the top of the metal due to the various actions of the gasses present in the air. The brightness of the metals can be restored by rubbing the surface of the metals with the help of sand paper, because by doing show the outer layer gets removed and the metal become shiny and bright again.

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