Copper metal slowly reacts with the oxygen in air to form reddish copper (I) oxide, , which is a compound that coats the surface of older pennies and makes them look tarnished. When such a penny is...

Copper metal slowly reacts with the oxygen in air to form reddish copper (I) oxide,
, which is a compound that coats the surface of older pennies and makes them look tarnished. When such a penny is placed in a solution of salt in vinegar, the copper (I) oxide acts as a base and reacts with the vinegar to form copper salts. This effectively cleans the penny. The copper salts can then be transformed back into copper metal when exposed to an iron nail.

Stir about half a teaspoon of salt into about half a cup of white distilled vinegar. Use a nonmetal container, such as a ceramic or plastic bowl. Dip a tarnished penny halfway into the solution and notice the rapid cleaning effect. Add at least a dozen tarnished pennies to the solution. As they get cleaned this increases the concentration of copper ions in solution. Sandpaper an iron nail to give it a clean surface and then rest the nail in the vinegar solution for about 10 minutes. Watch for the formation of copper metal on the nail. Are copper ions positively or negatively charged? What is the charge on the copper atoms in
? What is the charge on the oxygen? What must copper ions gain in order to transform back into a metallic form? What was the iron of the nail able to do for the copper ions?