Acid rain causes metal bridges, cars, even concrete buildings, to erode. Chemicals from vehicle exhaust and industrial pollution combine with water in the atmosphere to make rainfall acidic, often as sulphuric acid. When copper, which is used on roofs of older buildings, is exposed to acid rain, it turns green.
We're going to simulate what happens, using a penny and some weak acid. The acid we're going to use is acetic acid, commonly known as vinegar.
You'll need a resealable sandwich bag, paper towel, a little vinegar, and a penny.
Wrap the penny in the paper towel, soak it in vinegar, and put it in the sandwich bag. Seal it up, and leave it for a day. Then take the penny out and look at it.
 The penny has turned green! The acetic acid combined with the copper in the penny to make a new substance, called copper acetate. Copper acetate is green. Surprisingly, this example of corrosion is not as harmful as other types, like rust on steel, for example, because the coating of copper acetate on the outside of the copper penny protects the rest of the penny from further erosion!
 Unlike rust, which flakes off, and leaves more of the metal on your car exposed to corrosion, copper acetate sticks to the copper and protects it!
This is why copper makes a good roof ... once the outer layer of copper on the roof corrodes, and turns to copper acetate, it forms a protective layer; the roof becomes impervious to further corrosion!
The roof of the Parliament Buildings in Ottawa, for example, is made from copper, and was originally brown. But we've come to love its corroded green colour.
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