Give a scientific reason for why lemon or tamarind is used for cleaning copper vessels that turned greenish.

Copper vessels turn greenish when they react with the carbon dioxide in the air to form copper carbonate. Hence, copper vessels lose the shiny brown surface. Now, the citric acid found in the lemon or tamarind, neutralizes the basic copper carbonate, thus dissolving the layer. This is the reason why tarnished copper vessels cleaned with lemon or tamarind helps the copper vessels to retain their lustre.