---layout: posttitle: Why Does Copper Oxidize and Turn Greenauthor: Joe Schwarczsource: McGill Blogs---Why Does Copper Oxidize and Turn Green?The natural color of copper is peachy or pinkish-white, with a brightmetallic luster. It is one of the very few elements, whose natural coloris neither white nor gray. However, it undergoes oxidation reaction andturns green after being exposed to air.HYPERLINK"https://www.facebook.com/sharer/sharer.php?u=http%3A%2F%2Fsciencestruck%2Ecom%2Fwhy%2Ddoes%2Dcopper%2Doxidize%2Dturn%2Dgreen" \t "_blank"HYPERLINK"https://twitter.com/intent/tweet?url=http%3A%2F%2Fsciencestruck%2Ecom%2Fwhy%2Ddoes%2Dcopper%2Doxidize%2Dturn%2Dgreen" \t "_blank" HYPERLINK"https://plus.google.com/share?url=http%3A%2F%2Fsciencestruck%2Ecom%2Fwhy%2Ddoes%2Dcopper%2Doxidize%2Dturn%2Dgreen" \t "_blank"Fast FactA prime example of copper oxidation is the Statue of Liberty, wherein agreen color has developed on the structure's surface due to redox, andnot due to effect of acid rain, as generally believed by people.According to the Copper Development Association, until now, theoxidation of the statue's copper skin is about 0.005 inch.Copper is a metallic element with atomic number 29, and is representedby the symbol 'Cu' (Cuprum). It is a malleable (can be shaped into athin sheet) and ductile (can be shaped into a thin wire) element. Inhumans and animals, it is an essential metal that is found in the bodycells and tissues. Due to high thermal and electrical conductivity,copper is used in many aspects of life, such as building materials,thermal and electrical conductors, and in the manufacture of variousmetallic alloys. In fact, its use can be traced back to the ancienttimes and early civilizations. This is evident from the old coppercrafts, copper wares, and other material remnants of the pastcivilizations. Due to its chemical properties, copper ranks third as anindustrial metal, next to aluminum and iron. In nature, copper isusually found in association with sulfur, which is then purified bymeans of other techniques.Examples of Oxidationโ Oxidation of iron is referred to as 'rust', and it leads to thecorrosion and formation of a flaky, reddish-brown outer layer.โ Another example is the oxidation of aluminum, leading to thedevelopment of a flaky white layer. This is usually seen on aluminumdoors and windows after heavy rainfall.โ When an apple is cut and the surface is exposed to air, it turnsbrown.Oxidation of Copper and Changing of ColorOxidation is a phenomenon, whereby an element loses electrons (and/orhydrogen) on interacting with another element. Similar to iron andaluminum, the element copper undergoes the process of oxidation, if itis exposed to air. If the atmosphere consists of high humidity moisture,then this process is faster. The copper metal reacts with oxygen,resulting in the formation of an outer layer of copper oxide, whichappears green or bluish-green in color. This layer is known as thepatina.Unlike other destructive oxidation processes, the patina acts as aprotective layer, and it does not cause any weakness in the metal. Thus,copper is considered as an important element that is resistant tocorrosion. The patina prevents further corrosionoโฆๆฝฃ็ฐ็ฅๆ ๆนฅๆ ฅๆกด็ ๆจๆผ ๆฅธๆฅคๆบโคๆ ฌๆนโนฒๅ ็ฅโนๆฏๆดโฑฎ็ ๆฅจโณๆ ฌๆนโฒ็ฉ็ ๆฅโฎๆนฏ็ ๆจ็ ๆฝฏ็ฆ็ฏโณๆฏๆผ ๆฌๆ ๆฅตๆฌๆนฉ็งโฌๆนกโคๆก็ดๆ โณโกๆ ทๆด็ฒๆฝฒๆฏๆนฉโงๆนกโค็ณโตฎ็ฐๆฝฏๆฅฆๆฎๆ ๆ ฏโนดๅๆ ฅ็ฃๆฝฉ็ฎโ The oxidation process is characterized by the formation of threemain products: azurite, malachite, and brochantite. These mineralsimpart the characteristic bluish-green color to oxidized copper metaland form the patina layer. Simply speaking, it is a redox reactionwherein both oxidation and reduction take place at the same time.โ Firstly, copper donates electrons while reacting with oxygen to formcopper oxide, which is reddish brown in color. This can be representedas:4Cu + O2 = 2Cu2O (copper oxide) ---- equation 1โ Cu2O again reacts with oxygen to form CuO, which is black in color:2Cu2O + O2 = 4CuO ---- equation 2โ The atmosphere also consists of various pollutants like sulfurdioxide, carbon monoxide, carbon dioxide, etc. In areas where combustionof coal takes place, sulfur fumes are present on a large scale, assulfur is an impurity present in most of the coal types. Copper oxidereacts with this element to form blue-colored copper sulfate (CuS),which may further react with carbon dioxide and moisture present in airto form the patina layer.Alternatively, the copper oxide may also directly react with CO2, H2O,and sulfur oxides to form the compounds brochantite, malachite, andazurite:โ Brochantite is mainly formed when high level of sulfur fumes arepresent. It is green in color, and is also known as hydrated coppersulfate.4CuO + SO3 + 3H2O = Cu4SO4(OH)6 (brochantite) ---- equation 3โ Malachite is a green-colored, naturally formed compound, and alongwith azurite, it comprises greenish-blue colored, semi-precious mineralstones known by the same names. This indicates that oxidation of copperalso takes place in naturally formed rocks and gems, apart from copperutensils, statues, rooftops, etc.2CuO + CO2 + H2O = Cu2CO3(OH)2 (malachite) ---- equation 4โ Azurite is also known as hydrated copper carbonate, and it imparts aslight bluish tinge to the oxidized metal. It is formed near areas thatare mined for copper.3CuO + 2CO2 + H2O = Cu3(CO3)2(OH)2 (azurite) ---- equation 5How to Oxidize Coppertvwโฌยฌยฝwre several methods to oxidize copper at home using daily essentials likesalt, kitchen ammonia, baking soda, water, etc. By this technique, youcan impart an antique or old-style feel to your desirable copper items,like coins, medals, showcase pieces, etc.โ Firstly, clean the copper metal with mild detergent and water, andwipe evenly in a single direction.โ In a steel utensil, mix about half tablespoon salt, half tablespoonbaking soda, and about 15 oz. of ammonia in half liter of water.โ Fill this solution in a spray bottle and spray the copper item atleast once in a day. After few weeks, a green-colored patina will startcovering the metal surface.โ It is better to perform this technique outdoors to get smoother andfaster-developing carbonate layers.โ To oxidize smaller objects like copper coins, a simpler method is tohang them in a jar without touching the sides and the bottom.โ They should be cleaned with alcohol and washed with detergent beforeattaching them to the jar with a thread.โ The jar bottom is filled with half a centimeter of ammonia, and asthe fumes start reaching the copper surface, close the jar with a lid.โ In a couple of hours, a clear green layer starts forming on theirsurface. Unscrew the lid and remove the coins after a desirablethickness of patina has been developed.For preventing oxidation, the best way is to protect the surfaces, whichare likely to be exposed to air and water, with the help of a protectivecoat. This way, the metal surface is prevented from reacting with oxygenand/or water. You can also apply lemon on the copper surface to removeany patina layer of carbonates that have formed on the oxidized items.ls, showcase statues, pipes, etc., is to immerse them into a solutionconsisting of water, vinegar, and table salt. Heat the water tillboiling point, and leave the items immersed till a couple of hours.After removing them, clean the items with soap or detergent to get backthe shiny copper coat.