Copper turns green because of a process known as oxidation which is the removal of electrons from the substance. Specifically, copper turns green because of something known as copper carbonate. This is the substance that is found on top of copper–whether they be copper pipes, pennies, statues or anything else. So, the cause of copper turning green is copper carbonate. However, the big question is: why does copper carbonate appear?
When a metal comes into contact with air or water, it undergoes environmental changes. In other words, it might rust or it might tarnish. Copper is a metal that tarnishes and turns green. Iron rusts because rust is iron oxide. When you think of what causes iron to rust–rain and wind–you can understand easier what causes a copper penny, for example, to rust.
An important thing to consider, though, is that oxidation is a process. The penny, for example, won't turn green overnight. However, if it gets wet, nothing will happen for a little while. As time goes on, though, it might start turning brown and then black. And then, after a while, shades of green will begin appearing. These shades of green are the build up of copper carbonate.
It should be noted, though, that copper carbonate does not weaken the metal. Unlike iron that grows weaker as it rusts, since the copper carbonate is only a tarnish, it doesn't take away from the strength of the copper. Therefore, a green copper pipe is no weaker than a cleaned copper pipe. It's just unappealing.
Removing the green from copper can be done with a few different ways. The first is to rub lemons over the copper. This will allow the break up of the copper carbonate to take place. Another thing to do is boil water, put a tablespoon of vinegar and salt into it and then put the objects that are green in there. Allow them to boil for a few hours and then remove them and wash with soap and water. This will remove any of the green tarnish.
Acid Rain Does Not Cause Copper to Turn Green
Despite many people assuming that it does, acid rain does not cause copper to turn green. People usually ask this after seeing the Statue of Liberty which is a copper statue. However, the truth is, as the statue was bombarded with rain and air, it simply underwent oxidation. No amount of acid in the rain would have caused the copper to turn green.