Why does copper turn green?

Like certain other metals, it oxidizes when left out in the elements, but the coloring process is intricate.

 

Copper has a gorgeous reddish tint, but when exposed to the elements, the metal undergoes a sequence of chemical processes that make it turn green.

 

But why does this hue shift occur? The solution, it turns out, is identical to why iron rusts; if iron is left unprotected in the open air, it will rust and create a flaky orange-red outer layer.

 

"When copper metal corrodes, it generates what is termed an oxide layer," Paul Frail, an advanced senior engineer in corrosion treatments with Suez Water Technologies & Solutions based in Trevose, Pennsylvania, told Live Science.

 

The oxide layer, Frail explained, arises when the surface of copper combines with the oxygen and water found in the Earth's atmosphere. The coating is made up of copper salts and oxygen and becomes thicker with time. Eventually, the copper behind the layer is no longer exposed enough to the air to react.

 

"Initially, the film may seem tarnished or black," said Frail, who is also a member of the American Chemical Society. "As the oxide coating matures and grows more color, it will begin to [alter], ranging from yellow-reds to blues and to a greenish color."

 

He noted that the Statue of Liberty is an outstanding example of copper turning green, as is a copper metal used in other sorts of monuments and older structures for government, offices, and institutions.

 

The color we see on older copper exposed to the air is not directly attributable to copper oxide or the reaction of copper with oxygen in dry air, said Mark Jones, a retired chemist with Dow Chemical.

 

When the oxide reaction happens, the oxides are not colored. Rather, the color originates from "reactions of residues of sulfate and chloride in the environment with the copper oxide," Jones told Live Science. Sulfur oxides occur from the combustion of fuels with sulfur, for example, and then fall onto the copper through the rain.

 

"They react with the oxides on the surface [of the copper] and provide color," Jones said of the sulfur oxides, which are always present at low levels in the air. This is one instance of how copper's slow color shift involves numerous phases.

 

On the periodic table of elements, copper is positioned next to nickel and zinc in the first row of what are called transition metals, which refer to metallic metals with certain qualities.

 

These features include being good conductors of electricity, being resistant to corrosion, being exceedingly bendable (or shapable), and serving as good conductors of heat.

 

Frail noted that copper, like these other metals, can easily be combined to make alloys, which have desirable features in building, including slow corrosion when compared with iron. "A common alloy of copper is brass, where copper is combined with zinc," Frail remarked.

 

According to Jones, the fact that copper is located above silver and gold on the periodic table indicates that it has a chemistry that is comparable to that of these elements. He made the observation that none of them oxidize quickly. While gold is entirely resistant to oxidation, silver is only slightly less resistant than gold, and copper is even less resistant than either gold or silver.

 

Copper's usage in electrical applications is further supported by the fact that it possesses all of these characteristics, in addition to having a higher natural abundance than both gold and silver. Copper is also the primary ingredient in the catalyst that is utilized during the production of methanol and vinyl chloride.

Article source : https://www.livescience.com/32487-why-does-copper-turn-green.html

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