Reaction:
2Cu (s) + H2S (g) + O2 (g) → CuS (s) + H2O (l)
- Copper sulfide is a dark-colored compound that forms when copper reacts with hydrogen sulfide and oxygen.
- Old mirrors often contain a thin layer of copper on the back to provide a reflective surface. Over time, the copper can react with gases like hydrogen sulfide and oxygen present in the air, leading to the formation of dark copper sulfide spots.
2. Silver tarnishing:
Reaction:
4Ag (s) + O2 (g) + 2H2O (g) + 2CO2 (g) → Ag2CO3 (s) + 2H2O (l)
- Silver tarnishes when it reacts with oxygen, moisture, and carbon dioxide to form silver carbonate, a dark-colored compound.
- Mirrors often have a thin layer of silver on the back to provide a reflective surface. Prolonged exposure to the environment can lead to the tarnishing of the silver layer and the appearance of dark spots.
3. Mercury oxidation:
Reaction:
2Hg (l) + O2 (g) → 2HgO (s)
- Mercury is a liquid metal used in older mirrors. Over time, mercury can react with oxygen in the air to form mercury oxide, a dark-colored compound. This can result in the formation of dark spots on the surface of the mirror.
4. Atmospheric contaminants:
- Dark spots on old mirrors may also be caused by the accumulation of dust, dirt, and other environmental contaminants on the surface.
5. Paint or backing deterioration:
- In some cases, the dark spots may be due to the breakdown or deterioration of the paint or backing materials used in the construction of the mirror.