Question

why turnished copper utensils are clean with lime juice or tamarind

Answer 1

Copper metal slowly reacts with water, carbon dioxide and oxygen present in air to form basic copper carbonate which is green in colour. Its layer slowly gets deposited on the surface of the metal.
Now as lemon is acidic, hence when we rub the tarnished copper vessel with lemon, the basic copper oxide or copper carbonate reacts with the acid present in lemon to form a salt which is washed away with water..
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