Why Does Copper Turn Green?

Though copper is well known as a metal that resists the effects of exposure to the air and to salt water (because of the salt), it does have an interesting characteristic when this exposure occurs. It turns green. Even so, it is such a durable and workable material that it has been used for many purposes for thousands of years.

copperThis color comes from a very thin coat that forms due to exposure. The coating is called by the chemical name copper carbonate. Structures such as the Statue of Liberty exhibit this, as do other structures and copper roofs. According to scientific texts and research, the oxidation that turns copper green is caused by the basic elements losing electrons or reacting to some specific elements. Even fruit shows this change when sliced or cut.

Copper undergoes this basic chemical change due to a process called oxidation. The process is widely recognized as causing various changes in materials that are exposed to air, wind and rain. All metals show this change, though copper is one of the metals that exhibits a change not really considered deterioration. Iron, for example, rusts from oxidation and is believed to become less stable and valuable because of it.

The green color is a “patina” that actually protects copper. But close study has shown that this color doesn’t appear right away. Copper tends to show a dark brown or black color when oxygen and water first start to act on it. That changes with time. Some people have even worked with copper to speed up or control the color change to achieve certain appearances.

Of course, with other metals, the result is not so “beautiful” or “desired.” When iron rusts due to oxidation it flakes and pieces of the surface fall off. When aluminum rusts the flaking is white, but it still seems to be deterioration. The result with copper isn’t considered quite so negative.

Copper doesn’t react this way when it comes into contact with fresh water (water that contains no salt). With its protection of the patina copper is great for use in certain construction settings. It resists the corrosion that affects iron, aluminum and some other metals. The coating is even considered to be waterproof.

Copper has another characteristic that, when combined with the interesting color change, makes it popular with builders, engineers and scientists. It is valued as a construction and industrial metal only behind iron and aluminum.

The patina on copper is green, as mentioned earlier. The color green shows when the spectrum of light is in a certain wavelength. It’s not considered a primary color, but is a combination of blue and yellow. It’s interesting to note that the name for this color comes from a word that meant “grow” in Old English. The fact that the patina is green and is considered protective associates in an interesting way with the history of the word.

Category: Chemistry, Science

2 Comments on “Why Does Copper Turn Green?”


Kevin smith wrote:

I just bought an old copper fire extinguisher and it’s oxidized pretty bad, am I going to be able to restore it back to a new condition?

maxwell wrote:

what is the chemical equation of this reaction?

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