Question

copper substances when exposed to air gains a green coating due to the formation of

Answer 1

copper oxide
copper reacts with atmospheric oxygen to becom copper oxide

Answer 2

Explanation:

When copper substances when exposed to air it get oxidized and forms copper hydroxocarbonate (green in color).

Copper reacts with water vapors, carbon-dioxide and oxygen to give green color compound of copper hydroxocarbonate which is insoluble in water..

$2Cu + O_2 + H_2O + CO_2\rightarrow Cu_2(OH)_2CO_3$