Reena 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.

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Solution

The phenomenon is called corrosion. The black coating is silver sulphide (Ag $_{2}$ S) formed due to the reaction of H $_{2}$ S gas present in the atmosphere with silver and the green coating is basic copper carbonate formed due to the reaction of moist air (CO $_{2}$ , O $_{2}$ and H $_{2}$ O vapours) with copper.
$2 A g S i l v e r + H_{2} S H y d r o g e n s u l p h i d e \to A g_{2} S S i l v e r s u l p h i d e (B l a c k) + H_{2} H y d r o g e n$
$2 C u C o p p e r + C O_{2} + O_{2} + H_{2} O F r o m m o i s t a i r \to C u C O_{3} + C u (O H)_{2} B a s i c c o p p e r c a r b o n a t e (g r e e n)$

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