Why Does Copper Oxidize and Turn Green

Copper is a metallic element, having atomic number 29 and represented by the symbol, Cu. It is malleable (can be made into a thin sheet) and ductile (can be made into a thin wire) element. In humans and animals, copper is an essential element found in the body cells and tissues.

Due to high thermal and electrical conductivity, copper is used in many aspects of life such as building material, thermal and electrical conductor and in manufacturing other metallic alloys. In fact, the use of copper element can be traced back to ancient times and the early civilizations. This is evident from the ancient copper crafts, copper wares and other material remnants of those times. Due to its chemical properties, copper ranks third as an industrial metal, next to aluminum and iron.

In nature, copper is usually found in association with sulfur, which is then purified by means of other techniques. The natural color of copper is peachy or pinkish, with a bright metallic luster. It is one of the very few elements, whose natural color is neither white nor gray. However, the color of copper turns green after exposure to air and moisture. Let's take a look as to why copper oxidizes and turns green.

Why Does Copper Oxidize and Turn Green

Oxidation is a phenomenon whereby an element loses electrons (and/or hydrogen) on interacting with another element. There are many examples of oxidation, which we observe in our day-to-day life. To mention a few; there is oxidation of iron, aluminum and even, a freshly cut apple.

Oxidation of iron, referred to as 'rust', leads to corrosion and/or formation of a flaky, reddish outer layer. Another example is the oxidation of aluminum, leading to formation of white, flaky layer. This is usually seen in aluminum doors and windows after a heavy rainfall. When an apple is cut and the cut surface is exposed to air, it turns brown. This is nothing but oxidation.

Similar to iron and aluminum, the element copper undergoes the process of oxidation, if it is exposed to air. Over here, copper metal reacts with oxygen, resulting in the formation of copper oxide, which appears green in color. It is to be noted that copper does not react with water. The outer green layer, formed after the oxidation of copper, is known as patina.

Unlike other destructive oxidation, patina acts as a protective layer. This is the reason as to why, copper is considered as an important metal, resistant to corrosion. The patina prevents further corrosion of copper beneath the oxidized layer. Very often, patina is seen on the roofs of old buildings, which acts as a waterproofing layer.

The function of patina as a protective layer can also be seen in the Statue of Liberty. According to the Copper Development Association, the oxidation of the statue's copper skin is only about 0.005 inch till now. For preventing oxidation, the best way is to protect the surfaces, that are likely to be exposed to air and water, with the help of a protective coat. This way, the surface of the metal is prevented from reacting with oxygen and/or water.