What happens when copper vessel is exposed to moist air for a long time? State.

When a copper vessel is exposed to moist air for a long time, it forms a green layer on its surface. This green layer is a mixture of copper carbonate and copper hydroxide. The reaction occurs because copper reacts slowly with water, oxygen, and carbon dioxide present in moist air. 

This process is a type of corrosion. It makes the copper vessel look dull and reduces its shine. To restore the original shine, people often clean it with acidic substances like lemon juice or vinegar. This chemical reaction can be written as:

Copper + Water + Carbon dioxide + Oxygen → Copper carbonate + Copper hydroxide (Green layer). This is a slow change and is an example of a chemical reaction that changes the appearance of copper.