Methods of copper oxidation

How it reacts with air

Cop­per ox­i­da­tion is a nat­u­ral process. Cop­per is an el­e­ment that be­longs to the group of met­als and holds 29th place on Mendeleev’s pe­ri­od­ic ta­ble. Cop­per, like all oth­er met­als, is ca­pa­ble of ox­i­da­tion, form­ing sta­ble bonds in the forms of ox­ides and salts. To un­der­stand what cop­per ox­ide is, you can look at an old bronze stat­ue which has been stand­ing for many years. The stat­ue will be of a light green col­or, be­cause the cop­per con­tained in bronze has ox­i­dized. A new bronze stat­ue will be the col­or of brick, but from moist air and car­bon diox­ide, over time the fol­low­ing re­ac­tion takes place:

2Cu + H₂O + CO₂ + O₂ → Cu­CO₃•Cu(OH)₂

This com­bi­na­tion of cop­per salt and hy­drox­ide is called mala­chite, which is used to make paints and jew­el­ry:

Malachite jewelry box [Deposit Photos]

There are many meth­ods of ox­i­diz­ing cop­per items. In in­dus­try, ox­i­dized cop­per is made by an­od­ic ox­i­da­tion, us­ing elec­trodes. This method, which re­quires spe­cial equip­ment, is a com­plex and ex­pen­sive process. Ox­i­da­tion at home is much more straight­for­ward.

Cop­per ox­i­da­tion at home

To un­der­stand ful­ly what cop­per ox­i­da­tion is, we can con­duct an ex­per­i­ment. It’s not dif­fi­cult to make cop­per ox­ide at home. We will need the fol­low­ing:

So, hold­ing the cop­per wire with the pli­ers on one side, we put the free end of the wire into the flame of the burn­er and heat it. Thanks to the high burn­ing tem­per­a­ture, over time the wire turns black. In the chem­i­cal re­ac­tion process, cop­per trans­forms into cop­per ox­ide:

2Cu + O₂ = 2CuO

Then we put the cop­per wire in a hy­drochlo­ric acid so­lu­tion. We im­me­di­ate­ly see that the so­lu­tion changes from col­or­less to turquoise. This col­or is char­ac­ter­is­tic for cop­per chlo­ride, which is formed in the re­ac­tion process:

CuО + 2HCl = Cu­Cl₂ + H₂О

Note that the cop­per wire has once more turned a light brick col­or.

Unoxidized copper wire (left) and oxidized copper wire (right) [Wikipedia]

The black cop­per wire is also re­duced us­ing ethyl al­co­hol. Put it into a flask of ethyl al­co­hol, and the cop­per item will once more turn a gold­en col­or. Thus, as a re­sult of a com­plex chem­i­cal re­ac­tion, ethyl al­co­hol ox­i­dizes to acetic alde­hyde. Click here, for do­ing amaz­ing ex­per­i­ments with cop­per.

To turn the dark­ened cop­per wire back to its orig­i­nal col­or, we can also use a pow­der in the form of am­mo­ni­um chlo­ride.

[Deposit Photos]

We need to cre­ate the fol­low­ing chem­i­cal in­ter­ac­tion:

CuО + 2N­H₄­Cl = 2NH₃ + H₂О + Cu­Cl₂

We put the hot cop­per wire on the bot­tom of an earth­en­ware bowl con­tain­ing am­mo­ni­um chlo­ride (NH₄­Cl). As a re­sult of the re­ac­tion, am­mo­nia gas will be re­leased, and the dark­ened cop­per wire will once more turn gold.

Dur­ing chem­i­cal ex­per­i­ments, you should fol­low safe­ty rules when work­ing with a gas or spir­it burn­er, and also with ag­gres­sive sub­stances such as hy­drochlo­ric acid.