Question

Compare what happens when two groups of copper pennies are exposed to the oxygen in air. One group of pennies will be rinsed in water, while the other group will still be coated in a vinegar and salt solution.

Answer 1

I assume newly minted pennies, with a fresh copper surface exposed. In that case, the first group would be expected to slowly darken as black copper (II) oxide is formed at the surface from the reaction of the copper and atmospheric oxygen. In the second case, however, I would expect the more rapid formation of verdigris, which is a complex mixture to which copper (I) acetate lends a bright green color. The salt will help this electrochemical reaction proceed, as it makes the solution conductive.