Have you ever noticed the difference between an old and a new penny? The newer pennies look so much shinier than the old ones! Why is that? You might think the old pennies just have dirt and grime built up on them over the years, but they are also covered with copper oxide. Remember last week when we learned about atoms? (If not, check out last week’s blog HERE).
Everything in the world is made up of atoms! Sometimes, atoms combine with one another to make new materials. Copper atoms can combine with the oxygen in the air to make copper oxide. Copper oxide is what makes older pennies look so dull.
As you can see in the image below, the 1995 penny is much shinier than the 1944 penny.
YOU WILL NEED:
- Large, non-metal bowl
- 1/4 cup white vinegar
- 1 teaspoon salt
- Pennies (make sure they are dull ones, not shiny ones)
- Nuts and bolts (screws or nails also work, but be careful!)
- Paper towel
- Clock or timer
Here’s what to do!
- Pour 1/4 cup white vinegar into the bowl.
- Add 1 teaspoon of salt into the vinegar and mix it up until the salt completely dissolves. You now have a vinegar-salt cleaning solution.
- Place about 10 pennies in the bowl of solution. Set your clock or timer for 10 seconds.
- Remove all 10 pennies from the solution and set 5 of them on the paper towel marked “not rinsed.”
- Rinse the remaining 5 pennies off with water and set them on the paper towel labeled “rinsed.”
- Wait 1 hour before you come back to the pennies. While you’re waiting, place a few nuts and bolts into the same bowl of solution that you had the pennies in and set the timer for 10 minutes. What happened to the nuts and bolts? They changed color! This happened because when you dissolved the copper oxide in the vinegar-salt solution, some of the copper atoms dissolved with it. When the copper atoms separated from the penny, they left some of their electrons behind, so they went from neutral copper atoms to positively charged copper ions. When you put the nuts or bolts into the solution, the same thing happened to the steel. Some of the steel atoms left the nuts and bolts and became positively charged steel ions. Because the nuts and bolts now have an excess of electrons, they became negatively charged. What do you know about opposite charges? They attract! The negatively charged nuts and bolts attract the positively charged copper ions that are floating around, so they end up coating the nuts and bolts with a thin layer of copper.
Your results should look similar to the image below:
8. Now look at the unrinsed pennies. What do they look like? Very different from the rinsed pennies! Record the changes in your observation journal.
What happened to the unrinsed pennies? They turned a bluish-green color! You should have gotten a result somewhat like the image below.
Now that you know vinegar and salt work as a cleaning solution to dissolve the copper oxide off of pennies, try other acids to see if they work just as well! For example, you could try using lemon juice to see if it works better or worse than vinegar. What other acidic liquids can you think of? You could also adjust the amounts of salt to see if that makes a difference. Remember to record your findings in your observation journal! Happy experimenting!