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.
Tarnished copper vessels develop a green coating of basic copper carbonate when exposed to moist air containing carbon dioxide. This green substance forms because copper reacts with moist carbon dioxide in the air.
Lemon juice and tamarind juice contain acids (citric acid in lemon, tartaric acid in tamarind). When these sour substances are applied to tarnished copper vessels, the acid reacts with the basic copper carbonate coating and neutralises it. This chemical reaction dissolves the green coating, exposing the shiny copper surface underneath.
The acidic substances thus effectively remove the basic carbonate layer and restore the shine of the copper vessel.
Explanation
The chapter mentions in Section 3.5 that copper reacts with moist carbon dioxide in the air and slowly loses its shiny brown surface and gains a green coat of basic copper carbonate. Since this green substance is basic in nature, acidic substances like lemon juice or tamarind juice can neutralise and dissolve it through an acid-base reaction.