Question

Why are sour substances/like lemon or tamarind juice used for cleaning tarnished copper vessels?

Answer 1

In tarnished copper vessel there is layer formed due to corrosion which is basic in nature which is dissolved in the lemon or tamarind juice which are acidic in nature. Thus the vessel get thoroughly cleaned.
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Answer 2

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Copper reacts with moist carbon dioxide in air to form copper carbonate and as a result, copper vessel loses its shiny brown surface forming a green layer of copper carbonate. The citric acid present in the lemon or tamarind neutralises the basis copper carbonate and dissolves the layer. That is why, tarnished copper vessels are cleaned with lemon or tamarind juice to give the surface of the copper vessel its characteristic lustre.