The correct option is Basic copper carbonate.
Explanation
The phenomenon described is the corrosion of copper. When copper is exposed to moist air for an extended period, it reacts with atmospheric gases to form a layer often referred to as patina.
Detailed Analysis:
- Formation of Green Coating: Copper reacts with carbon dioxide ($CO_2$), oxygen ($O_2$), and moisture ($H_2O$) present in the air. This reaction causes the metal to lose its shiny brown surface and develop a green coating.
- Chemical Composition: This green substance is basic copper carbonate. It is chemically a mixture of copper carbonate and copper hydroxide.
- Chemical Equation: The reaction is represented as:
$2Cu + H_2O + CO_2 + O_2 \rightarrow Cu(OH)_2 + CuCO_3$ (Basic Copper Carbonate) - Incorrect Options:
- Copper Oxide ($CuO$): This is a black oxide formed when copper is heated strongly in the presence of oxygen.
- Copper Sulphide: This is typically black and is not the product of the reaction with moist carbon dioxide.
Key Takeaway: The green coating formed on copper due to corrosion in moist air is basic copper carbonate ($CuCO_3 \cdot Cu(OH)_2$).