Answer the following question.

Why does a copper vessel shine again after scrubbing?

After a longer period of time, the copper vessels become tarnished when in contact with air. This layer is formed due to oxidation of copper in air. It is a greenish or blackish copper oxide coating which is basic in nature. To remove this coating, the vessel is rubbed or scrubbed with an acidic material ex lemon, tartaric acid. The reaction of basic oxide layer with the acid results in the formation of a salt, which can be easily washed away with water. And, the copper metal vessel shines again.