why tarnished copper vessels are cleaned with lime juice or tamarind
Copper vessel gets tarnished due to copper oxide layer formed on its surface.Copper oxide is basic in nature.Lemon or tamarind is acidic.Hence, when we rub tarnished copper vessel with lemon juice or tamarind, the basic copper oxider reacts with acid(citric acid) present in the lemon or tamarind to form a salt which is washed away with water.Hence,the vessel gets cleaned.
Copper + Carbon dioxide + Water + Oxygen -> Copper Carbonate + Copper hydroxide
The nature of its corrosive substance is basic (basic copper carbonate). Thus substances of acidic behaviour are generally used to neutralize the basic nature and dissolve the corrosive layer of basic copper carbonate. This evinces a clean and shiny brown layer (original) of copper.