You must have been seen tarnished copper vessels being cleaned with lemon or tamarind juice. Explain why those sour substances are effective in cleaning the vessel.

Explanation

Copper reacts with most carbondioxide 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 basic copper carbonate and dissovles the layer. Thast is why, tarnished copper vessels are cleaned with lemon or tamarind juice to give the surface of the copper vessels its characteristic luster.