The Clean Penny Chemistry Experiment

Discussion:

Pennies made before 1982 are composed of 95 percent copper. Over time, as the copper reacts with oxygen in the air, they become coated with copper oxide and take on a dirty or sometimes blue-green hue. Solutions like the vinegar and salt mixture (or ketchup, which includes salt and vinegar in addition to other ingredients) react with and dissolve the copper oxide.

If left unrinsed and sitting in open air, the vinegar-soaked penny will develop a blue-green copper oxide residue on the surface after just a few hours. This is because the vinegar and salt solution speeds the reaction between copper and oxygen. Fun fact: this same reaction is what gives the Statue of Liberty its green color.

Sources:

playfullearning.net

planet-science.com