Question

The copper vessel was getting black from outside it may due to

Answer 1

Oxidation of Copper by Moist air forming Copper oxide.
This may happen when the Copper metal is exposed to air. The moisture in the air reacts (or better we can say, oxidises) with the metal and causes it to turn black.
This leads to the formation of Black Copper Oxide layer on it.
Cu + H2O -----› CuO + H2

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Answer 2

Answer:

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