Let's start with an experiment
You will need the following materials:
1/4 cup lemon juice (or vinegar)
a pinch of salt
disposable cup (or another disposable container like a bottle or yogurt container)
iron nail (or paper clip)
20 pennies (dull and old is best)
a steel wool scouring pad (if you have one)
Procedure:
Step 1. Pour the lemon juice (or vinegar) into your disposable container.
Step 2. Place all the pennies in the lemon juice.
Step 3. Add a pinch of salt to your lemon juice. Observe the pennies for 3-5 minutes. What changes do you see happening?
Step 4. Rub the pointed end of your nail (or your paper clip) with the steel wool, if you have it. Rinse the nail or paperclip off with water.
Step 5. Place the pointed end of your nail (or your whole paperclip) into the lemon juice and salt solution with the pennies. Wait for at least an hour, but the longer the better.
Step 6. Carefully remove the nail (or paperclip) from the solution and rinse it off with water. What do you notice about the nail? How does it look different than before you put it in the solution? If it doesn't look different, put it back into the lemon juice and salt solution for another hour or two.
Note: If you are having a hard time with the procedure or are unable to follow the procedure for any reason, you can see an example of the experiment below. Keep in mind that the procedure in the video is slightly different than the one described above.
What's Going On?
You probably already know that pennies are coated with copper to give them that bright and shiny bronze color. (If you didn't know that, or are curious about how the making of pennies has changed over time, read the "What's a Penny Made of?" article linked below.) Over time pennies start to look dull and dirty because the copper slowly reacts with the oxygen in the air, forming a greenish copper oxide. When you first added the penny to the lemon juice solution, you should have noticed that it started to look clean. That's because the citric acid in the lemon juice dissolves the copper oxide!
After an hour or more in the container of lemon juice and pennies, you should have noticed that the nail started to turn a copper-color. You copper-plated your iron nail! When you put the pennies and nail into the acidic solution, the copper and iron atoms start dissolving. What this means is that the copper and iron atoms start to break off of the penny and nail, respectively, and float around in the solution. Both the copper and iron atoms in solution are positively charged, but the copper has a stronger attraction to the negative charge on the nail than the iron, causing it to form a copper-coating!
Now, you might be wondering why we added the salt. If not, you should be! The salt creates another chemical reaction that disrupts the chemical equilibrium, and adds positively charged hydrogen atoms to the solution. This helps speed up the chemical reaction. Don't believe me? You can test this later, so keep reading!
If you would like to see a visual model of the chemical reaction, and how this same process can be further sped-up with electricity in a process called electroplating, watch the video below. The second video explains chemical equilibrium.
Challenge/Extension:
How can you increase the speed and/or efficiency of the copper-plating reaction?
You have learned about the chemical reaction behind copper-plating iron with lemon juice (or vinegar). Can you use what you have learned to answer the question above? Consider setting up an experiment with different solutions you have available at home (e.g., water, ketchup, salsa, pickle juice, dish soap, fruit juice, soda, etc.). How acidic or basic are each of the solutions you chose? Alternatively, you could use one solution and change how much salt you add to see how much of an effect it really has.
Once you come up with a plan, gather your materials, and set up your experiments. Make sure you collect data as you conduct your experiments! You may need to use a timer, or come up with a scale to describe how much copper-plating occurred.
If needed, watch the video below to review dependent, independent, and control variables before planning your own experiment.