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Question
You must have seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why these sour substances are effective in cleaning the vessels.
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Solution
Copper reacts with moist carbon dioxide in the air to form copper carbonate, which causes a copper vessel to lose its shiny brown surface and form a green layer of copper carbonate. The citric acid present in lemon or tamarind neutralises the base copper carbonate and dissolves the layer. That is why tarnished copper vessels are cleaned with lemon or tamarind juice to give the surface its characteristic lustre.
\[\ce{2Cu + \underset{(Present in air)}{H2O + CO2 + O2} -> \underset{(green)}{\underset{Basic copper carbonate}{CuCO3.CU(OH)2}}}\]
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