Why do pennies turn black?

Answer 1

Half-reactionsSince 1982 US cents have been made of zinc plated with a thin layer of copper. It took the Mint several years to stabilize the plating process to prevent reactions when the coins were exposed to air, dirt, skin oils, and such. Many early-date zinc cents exhibit black spotting and mottling because of those problems. Vinegar Reaction

• Ideas suggests that the acetic acid in vinegar reacts with the copper oxide that has formed on older pennies, and the resulting byproduct is copper acetate. The copper acetate then dissolves in the vinegar solution, therefore getting rid of the black color-producing agents and making it a bit shinier, although it will not appear as shiny as it once was with just vinegar.

Vinegar and Salt Reaction

• The vinegar in this solution does its job and removes the copper oxide, thus getting rid of the black color. If you place the pennies in the salt and vinegar solution, then take them out without rinsing them, the surface will turn greenish if given ample time to do so. This reaction may become apparent within a few hours or up to a week. Science Project Ideas calls this reaction verdigris; this name can reference many different copper compound reactions, including the one that the salt and vinegar mixture produces. Copper chloride is the name of the chemical reaction of mixing the copper and the sodium chloride from the salt. Copper acetate is the name for the reaction of the copper and acetic acid in vinegar. To prevent this copper reaction, just rinse the pennies after you take them out of the solution