When copper reacts with moist carbon dioxide (CO2) in air, it forms a green coating of which one of the following compounds?

When copper is exposed to moist air, it undergoes a slow chemical reaction with oxygen, water, and carbon dioxide. Initially, copper reacts with oxygen to form copper(II) oxide, which is black [1]. In the presence of moisture and carbon dioxide, this oxide further reacts to form a green coating known as a patina [2]. This green layer is primarily composed of basic copper carbonate, which includes compounds like malachite. In chemical nomenclature, this is referred to as cupric carbonate (specifically basic copper carbonate). While copper(II) oxide is black [1] and copper(I) oxide (cuprous oxide) is typically reddish, the characteristic green 'verdigris' or patina is the result of carbonate formation [1]. Therefore, the green coating formed when copper reacts with moist carbon dioxide is cupric carbonate [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Activity 3.9 > p. 41
2. [2] https://en.wikipedia.org/wiki/Copper(II)_carbonate