Copper doesn't turn yellow; instead, it develops a tarnish, which can appear in various colors including shades of brown, green, and even black. However, the question can be rephrased to "What causes the tarnish on copper which is sometimes perceived as a yellowish-brown color?" We can then answer. The tarnish on copper, often perceived as yellowish-brown, is due to the formation of copper salts through oxidation.
Copper Tarnish Explained
Copper tarnish is not a single substance but a collection of compounds known as copper salts. These salts form when copper reacts with various substances in the air and water. The process that results in tarnish is called oxidation and causes the pure copper to form a thin layer of chemical compounds on its surface which appear as a tarnish.
Factors Contributing to Copper Tarnish:
Here's a breakdown of the elements that can cause copper to tarnish based on the provided reference:
| Factor | Explanation |
|---|---|
| Airborne Oxygen | Oxygen in the air reacts with copper, creating copper oxides. |
| Carbon Dioxide | Carbon dioxide, also present in the air, can further react with copper oxides forming copper carbonates. |
| Sulfur Dioxide | Sulfur dioxide, an air pollutant, contributes to the formation of copper sulfides. |
| Hydrogen Sulfide | Hydrogen sulfide also creates copper sulfides, especially in areas with higher levels of pollution. |
| Chlorine & Carbonates in Tap Water | Tap water can contain chlorine and various carbonates that react with copper, further contributing to the tarnish. |
Understanding the Colors
The range of colors seen in copper tarnish - from yellowish-brown to green or black - depends on the specific copper salts formed. For example, copper oxides tend to appear reddish-brown, while copper carbonates often exhibit a greenish hue. The exact appearance depends on the unique mix of salts present on the surface of the copper. The initial tarnish may have a yellowish-brown hue which may account for the original question.
Practical Insights
- Prevention: Keeping copper clean and dry and away from these reactive substances will prevent or reduce tarnish.
- Cleaning: Tarnish can be removed with appropriate cleaning agents.
- Environment: In heavily polluted areas, copper will likely tarnish more quickly and intensely.
