When copper pennies are soaked in vinegar, a chemical reaction called oxidation occurs. Here's what happens:

* Vinegar's Acetic Acid: Vinegar contains acetic acid, a weak acid.

* Copper's Reactivity: Copper is a relatively reactive metal.

* Reaction: The acetic acid in vinegar reacts with the copper in the penny, forming copper acetate and hydrogen gas.

* Copper Acetate: Copper acetate is a blue-green compound that forms a thin layer on the surface of the penny.

* Hydrogen Gas: The hydrogen gas is released as bubbles.

* The Penny's Appearance: The penny will lose its shiny copper color and become duller, acquiring a greenish-blue patina.

In simpler terms: The vinegar eats away a tiny bit of the copper, making the penny less shiny and giving it a greenish color.

Additional Notes:

* Time: The reaction happens slowly, taking several hours or days for a noticeable change.

* Concentration: Stronger vinegar solutions will react faster than weak ones.

* Other Factors: Factors like temperature and the presence of other substances can also influence the reaction.

Safety: The reaction is generally safe, but avoid getting the vinegar on your skin or eyes. It's a good idea to do this experiment in a well-ventilated area.