Why Copper Turns Green

Copper is an element having chemical symbol Cu. It is brownish-red metallic element. It is one of the most extensively used metals. The atomic number being 29, copper is present among the transition elements in the periodic table,

Copper tends to form two kinds of compounds: first, cuprous, the copper used in it has one valence electron; second, cupric, it has two valence electrons. Cuprous compounds have a great tendency to oxidize into cupric, even by mere exposure to air. While cupric compounds are relatively stable.

Comparatively an inactive metal, copper corrodes slowly in air and water. The phenomenon becomes vigorous in the presence of weak acids, such as, carbonic acid, resulting in a porous, green, basic carbonate of copper. This green product that results from corrosion is known as verdigris or patina. It may also appear on copper alloys, such as brass and bronze in addition to pure copper.

Copper Why Copper Turns Green

‘WHY’ of the title explained:

Now the question as to why it does change its color is explained by the chemical reaction with oxygen found in open air. The product so passively formed is copper oxide having green color. This green layer initially formed over exterior surface is referred to as patina. Unlike iron, copper does not react with water.

The blue-green patina usually forms on copper in outdoor atmospheres. It is a complicated and slow process. At first naked Cu reacts with the oxygen in the air to form the pink oxide, in chemical terms: cuprite, Cu2O, with Cu+1 ions. This slowly oxidizes further to black oxide, chemical notation: tenorite, CuO, having Cu+2 ions. Sometimes the black sulfide CuS may also form.

In a moist atmosphere, the blackish layer passively reacts with sulfur dioxide and carbon dioxide of the air to form the patina, a mixture of 3 minerals now, brochantite, a green, hydrated copper sulfate, malachite, the green, hydrated copper carbonate and azurite, the blue, hydrated copper carbonate.

Copper atoms from the metal surface have been fully oxidized into cupric ions in above mentioned compounds. The proportion of the components and the rate of patina formation are directly proportional to humidity and the amount of air pollution. The places where coal has been used for centuries, brochantite is frequently found there, such as on older copper roofs and statuaries in European cities.

An interesting feature of copper corrosion is where rust corrodes patina protects. This is another reason why copper is held as a valued metal. Actually below the oxidized layer, patina doesn’t let corrosion develop any more beneath the green layer. That is why patina is also used as a waterproofing layer on the roofs of old buildings.

A well-known example of oxidation is the patina formed over the surface of the Statue of Liberty. Its copper surface has developed a green colored protective film to the exterior, about 0.005 inches thick. This green layer of patina is protecting Lady Liberty from being exposed to air and water. Oxygen and water is prevented from reacting with the metal any further through this layer of patina.

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