Teachers, here's a fun science activity from my science site you can share with your class on

President Lincoln's birthday:

This is a fun experiment! You can clean old and dirty Lincoln pennies and explore some of the properties of

metals.

Pennies get dull over time because the copper in the pennies slowly reacts with air to form copper oxide.

Pure copper metal is bright and shiny, but the oxide is dull and greenish. When you place the pennies in the

salt and vinegar solution, the acetic acid from the vinegar dissolves the copper oxide, leaving behind shiny clean pennies.

You need

. Dirty Lincoln pennies

1/4 cup white vinegar (dilute acetic acid ) and 1 teaspoon salt (Sodium chloride, NaCl)
1 shallow plastic bowl
water and paper towels

This is what you do:

Pour the salt and vinegar into the bowl and stir until the salt dissolves.
Dip a penny halfway into the liquid and hold it there for 10-20 seconds.
Remove the penny from the liquid. What do you see?
Place the rest of the pennies into the liquid.
What happens?
After 5 minutes, take half of the pennies out of the liquid and place them on a paper towel to dry.
Remove the rest of the pennies and rinse under the tap.
Place these pennies on a second paper towel to dry.
Wait an hour, then take a look at the pennies you have placed on the paper towels.

Rinsing the pennies with water stops the reaction between the salt/vinegar and the pennies.

They will slowly turn dull again over time. The salt/vinegar residue on the unrinsed pennies

promotes a reaction between the copper and the oxygen in the air.

The resulting blue-green copper oxide is commonly called 'verdigris'.

You can also use an eraser (and rubbing the coin works really well), a blob of tomato ketchup,

or coca cola instead of the salt and vinegar.

Try them and compare what happens to the pennies.