Science

Shiny Pennies

by Amy Collins July 18, 2018

You will need:

• 1 Gratnells mini tray or small tub
• 1 plastic peg
• 7 one penny pieces
• 1 teaspoon of table salt
• 60ml distilled malt vinegar (sufficient to half-fill the mini tray)
• 1 lolly pop stick to use as a stirrer
• A few paper towels or pieces of kitchen roll

What to do:

Activity 1.
1. Add the salt to the mini tray or tub.
2. Add the vinegar to the mini tray or tub.
3. Use the stirrer to give it a good mix until the salt dissolves.
4. Use the peg to dip one of the pennies half into the solution, leave it for 10-15 seconds and observe what happens closely. Compare the two halves of the penny. What has happened?
5. Lift the penny out, give it a rinse with some water and pop it on a paper towel to dry.
6. Use the same mini tray of solution for the next activity.

Activity 2 – does it work with more pennies?

1. Place five pennies in to the mini tray of solution you made for activity 1, leave them there for 3 minutes and observe what happens to them.
2. Remove the pennies from the solution (keeping the solution to one side), rinse the pennies in some water and pop them on a new paper towel to dry.
3. Compare the pennies side by side with the one that has not been treated with the solution and the one that has been half treated. What has happened?

Please note, the sequence of the video has been speeded up slightly so the change in the appearance of the pennies will not be quite as quick in real life.

What is happening?

Pennies get dirty over time because they are made of copper. The copper atoms that make up the penny react with oxygen atoms in the air and this reaction makes a molecule called copper oxide. Copper oxide is more dull and dirty looking than copper. As it forms on the surface of the penny, so the penny begins to look dull and dirty. New pennies (check the year printed on the shiny pennies in your purse) haven’t had chance to oxidise yet, so they still look shiny.

Copper oxide dissolves in the salt and vinegar solution. Vinegar is an acid, and it is this property that is essential to the cleaning reaction.

Other things to try (will require more pennies, a couple of nails and some other dirty coins to experiment with)…..

Activity 3.

Using the same mini tray of solution, add five pennies to the mini tray for 3 minutes, remove them and place them on to a new paper towel to dry without rinsing them in the water. Place this paper towel next to the one holding the pennies from activity 2. After about an hour, look at the pennies. What colour is the paper towel underneath the non-rinsed pennies?
What is happening?
When the copper oxide layer has been removed by the salt and vinegar solution, it is easier for the exposed copper atoms to react with oxygen in the air and chlorine left behind from the salt in the solution to make a blue-green compound called malachite. The pennies that have been rinsed have no salt left on them, so this reaction cannot happen.

Activity 4.

Using the same solution, place a steel nail, fully submerged, into the solution. Rest another nail on the edge of the mini tray so it is only partly submerged. After 10 minutes, have a look at the nails to see what has happened. Have they changed colour? If not, pop them back as they were in the solution and check again in an hour.
What is happening?

During activities 1-3, the copper oxide and some of the copper are removed from the pennies and dissolved in to the solution. The copper ions floating in solution are positively charged and attracted to the negatively charged iron in the nail, they stick to it and form a coating of copper on the surface of the nail.

• Does the solution work to clean other old coins, try 2p, 10p and £1?
• What happens if you gently warm the solution before adding the pennies?
• Can you make any other penny cleaning solutions with other common household ingredients? Tip: lemon and orange juice are also acidic, so you can try them in place of the vinegar.

Health & Safety

As with all Gratnells Learning Rooms What’s In My Tray Activities you should carry out your own risk assessment prior to undertaking any of the activities or demonstrations.