Question

What will happen when any type of copper vessel is exposed to moist air for a long time? State.

Answer 1

Hint: The lighter and warmer moist air and behind the front is lifted upward and "overrides" the colder air. As the air rises and then cools, and if enough water vapour condenses, widespread clouds and precipitation develop. A layer of thin clouds is occasionally observed quite a thousand kilometers beforehand of a surface front.

Complete answer:
When a copper vessel is exposed to moist air for an extended time it develops a green layer on its surface. Copper corrodes by oxidation during which it reacts with oxygen within the air to make oxide. Oxide then combines with $CO_2$ to form copper carbonate, which provides it a green color.
Copper forms a green deposit on its surface when exposed to air which is typically a mix of copper carbonate and copper hydroxide. This process is named corrosion of copper. The green material may be a mixture of copper hydroxide ($Cu(OH)_2$) and copper carbonate ($CuCO_3$).
The moist air contains oxygen which can react with copper to supply oxide. Generally the oxides are going to be in green color. This is often why the copper vessel is exposed to moist air for some time .It acquires a dull green coating.

Note:
Because the metal reacts with various chemicals and gases present within the atmosphere and it leads to the formation of copper carbonate which is green in color. The reactions are going to be as follows. As a result, the corroded coating of green color is copper carbonate which is basic in nature.