Question

copper vessels are coated with a greenish deposit on their surface.why?
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Answer 1

Because when copper is left open they react with the oxygen present in air and gets oxidised that's why the the copper vessels get coated with a greenish deposit on their surface.

Answer 2

copper vessels are coated with greenish deposit on there surface because when they exposed to moist air the copper hydroxide and copper carbonate is formed on the surface