What happens to copper in vinegar?

The hidden chemistry of cleaning old pennies

Have you ever dropped a handful of dull, grimy old pennies into a bowl of vinegar, only to watch them transform back to a bright, shiny copper color almost instantly? It feels like a neat little magic trick, but it is actually a textbook chemical reaction happening right in your kitchen.

Vinegar is a mild acid, and when it interacts with the tarnish and copper surface of the coins, a reaction takes place. As the copper reacts with the vinegar and oxygen, it creates a dissolved blue compound known as copper (II) acetate.

If you leave that bluish liquid sitting out in a dish and let the vinegar completely dry, the water slowly evaporates and leaves behind a cool surprise: tiny, dark black crystals. These are solid copper acetate crystals, giving you a front-row seat to everyday chemistry using just a few simple household items.