Final answer:
The green stains on copper are generally composed of a basic copper carbonate, represented by the formula Cu2(OH)2CO3. This patina acts as a protective layer preventing further corrosion, a process known as passivation. The blue-green color is a result of copper reacting with atmospheric components, which can also be observed in reactions where copper ions are in solution.
Step-by-step explanation:
The chemical formula for the green stains on copper, often referred to as a patina, is typically a basic copper carbonate (Cu2(OH)2CO3). However, a patina may consist of various compounds, such as copper sulfates, carbonates, and chlorides, which form under different environmental conditions. This green compound forms as a result of the copper reacting with carbon dioxide and moisture in the air. The presence of the green patina on structures like the Statue of Liberty is a classic example of passivation, where the layer prevents further corrosion of the copper beneath. Moreover, the formation of this protective layer signifies the process of corrosion having taken a new direction towards stabilizing the metal surface. Copper compounds impart different colors: for example, copper(II) ions produce a blue color in solution, indicating the presence of copper ions during the oxidation process as evidenced in various redox reactions.