Green pennies

You will need:

A saucer
Some paper towels
Vinegar
3-5 pennies

What to do:

Arrange the paper towels into a wad on your saucer.
Pour enough vinegar into the saucer to cover the paper towel.
Place the pennies on top of the wet paper towel and leave for a few hours.
Encourage observations; look at both sides of the pennies. The tops of the pennies turn green and the bottoms of the pennies stay copper coloured.

What’s going on?

Vinegar is an acid that has the chemical name of 'acetic acid'. Part of this acid combines with the copper of the pennies to form a green coating that is composed of copper acetate. Oxygen must be present for this chemical reaction to occur. Oxygen comes from the air, and this is why the tops of the coins turn green but the bottoms do not.