The part that was submerged in the solution is shiny copper. The portion that remained out of the solution remains dull with copper oxide tarnish (dark brown and/or green).Now put 20 pennies in the solution and let them sit for about 5 minutes.
Remove the pennies and...
Rinse 10 of the pennies off and let them dry.
Just let the other 10 pennies dry.
Rinsed pennies are on the left.
Now wait an hour or so....
(Rinsed pennies are now on top)
So, What Happened?
The vinegar/salt solution removed the copper oxide from the pennies, leaving a clean copper surface on each penny.
Over time, all of the pennies will become tarnished with copper oxide once again, as the copper reacts with oxygen in the air.
The vinegar/salt solution that remains on the pennies that were not rinsed speeds up the oxidation process, so in an hour you're left with pennies that look like this:
Presented at the 2003 New Jersey Science Convention.