Lemon juice is a natural weak acid. Place a small amount of lemon juice in a paper cup. Dip one penny, covered in brown, copper-oxide, into the lemon juice. Watch as the acid combines and removes the copper-oxide. The penny that looked dirty and brown, going in, will come out looking brand-new and shiny.
Home remedies use the acid in cola to clear sink drains and clean surfaces. Compare the acidity in cola and lemon juice by testing the ability of each to remove the copper-oxide on a penny. Fill one bowl with lemon juice and another with cola. Place a dirty penny into each bowl. Let the penny stand in the bowl for 10 minutes. Remove the penny and compare which acid was able to remove the most copper-oxide from the penny.
Observe how well salt and vinegar affect the copper-oxide on pennies during and after exposure. Mix 1/4-cup vinegar in a bowl with 1 teaspoon salt. Dip 40 pennies, covered in copper-oxide, into the salt-vinegar mixture. After soaking them for 10 minutes, take the pennies out. Rinse off 20 pennies with water and place them on a paper towel. Leave 20 pennies, unrinsed, on a paper towel. After one hour, look at the unrinsed pennies. The once shiny pennies will be covered in a blue-green color. After the salt-vinegar mixture removed the copper-oxide, the pennies were left vulnerable to the oxygen in the air; and when combined with the chloride from the salt, this created malachite. These pennies are now covered in blue-green malachite.
Dill pickles, oranges and lemons are all foods that are naturally acidic. Use a knife to carefully cut a slit in the side of each food item. Push a penny, covered in copper-oxide, into each food item, keeping half of the penny in and half out. Allow the penny to sit for 10 minutes. Take each penny out and compare the effect of the vinegar in the pickle, the lemon juice in the lemon and the orange juice from the orange in cleaning the copper-oxide off the penny. Depending on the amount of vinegar used in processing the pickles, typically, the pickle cleans the penny the best.