Materials:
table salt
ketchup
small bowl
spoon
paper napkin
dirty pennies or other copper objects

Directions: Combine equal parts salt and ketchup in a bowl. Mix together well with a spoon. Use a napkin to rub the dirty pennies with the solution. The tarnish on the pennies will rub off relatively easily.

Science: Tarnish on metal is caused by a chemical reaction between the metal and oxygen in the air. Metal can be cleaned through abrasion (which can damage the metal underneath) or through a chemical reaction. In this experiment, the salt is lightly abrasive, but a chemical reaction is also occurring . When an acid (as is found in ketchup) is combined with salt, Cl- ions form in solution. When applied to a tarnished penny, these ions attack the oxides in the tarnish without affecting the base metal, leaving a shiny finish.

Extensions: Try doing the same experiment but with soy sauce, lemon juice or vinegar instead of ketchup. Which solution works best?

NOTE: These solutions may not be safe for all metals.