Separating Copper and Zinc from a Penny
Most people are under the assumption that pennies are made of copper, and that is partially true. Before 1982, pennies were 95% copper and 5% zinc. Due to the fact that the price of copper was rising, the US Mint had to change the penny's composition to keep the cost of pennies under one cent. Now, they are 97.5% zinc and 2.5% copper! Because of this change, we can do a couple of cool experiments to extract the zinc from one penny and remove the copper from another. In other words, you're getting two experiments for the price of one in this Hub!
Copper has a melting point of 1984.32 °F (1084.62 °C), and zinc has a melting point of 787.15 °F (419.53 °C). Because of this stark contrast in melting points, we can heat up a penny, and the zinc will become molten long before the copper does. The only problem is that the zinc is contained by the copper jacket, so we have to create a way to allow the zinc to escape. I use a pair of Cutco Super Shears to cut a slit in the penny. You could also use a pair of tin snips or a hacksaw to cut a notch (see photo). This will have to be done for both experiments.
Extracting Zinc from a Penny
In this experiment, we will be removing the zinc core from a penny. Once the zinc core is removed, the copper is pretty damaged from the flame, so it isn't really worth saving afterwards. The first thing we need to do is gather our materials. They are as follows:
- A Post-1982 Penny
- A Ceramic Bowl
- A Torch (there are many options here. I have a Bernzomatic Micro torch, which is about $30. As far as I know, Tractor Supply Company sells "pencil torches," which are essentially the same thing, just a lot cheaper. Also, see the eBay ad at the bottom for pencil torch listings).
- Needle Nose Pliers (to hold the penny)
First, we need to fill our bowl with sand. We are using the sand as a way to collect the molten zinc. Sand (or silicon dioxide, SiO2) is very unreactive, and any sand that sticks to the molten metal flakes off pretty easily. Next, grab the penny with your pliers at the edge opposite of the cut that you made with your saw/tins snips/etc. Hold onto as little of penny as possible so you can heat up as much zinc as possible to recover from the coin.
While holding your penny over the bowl of sand, turn on your torch and begin applying heat to the penny. After about 30-40 seconds (depending on the maximum temperature of your torch) the zinc inside will become molten, and the penny will start to droop. The copper jacket is so thin that the penny loses the ability to retain its original shape and structure. Once you see the penny begin to shrivel, drag the slot that you made across the sand a few times, and the molten ball of zinc should come out with ease. Wait at least 10 minutes for the zinc to cool and solidify before handling. There you have it! Your own elemental zinc! See the video below for a demonstration of the process. You will see the penny start to droop at the 40 second mark.
Removing the Copper Jacket from a Penny
In this experiment, we will be dissolving the zinc out of a penny so that all you are left with is the copper coating. Unlike the previous experiment, the zinc will be all used up to obtain the copper. Your materials are as follows:
- A Post-1982 Penny
- Hydrochloric Acid
- Tweezers (for Penny Retrieval)
As with the previous experiment, you have to cut a slot in the penny to allow the zinc to be attacked by the hydrochloric acid. Making a few scratches on the surface of the penny to reveal the zinc would also be just as effective. Hydrochloric acid attacks and dissolves zinc at a much faster rate than it does copper. Because of this property, you could leave the penny in the acid for a couple of days, and the copper will still remain. I'm not entirely sure on the time it takes to fully dissolve the zinc, as I believe it will differ based off of ambient temperature, the types of cuts in the penny, and the exact percent composition of the zinc in the penny. All of the previous experiments I have done were left for 24 hours.
So, what is happening in this reaction in terms of a chemical equation? What are the bubbles that form as the reaction occurs? Let's take a look!
Zn (s) + 2HCl (aq) -> ZnCl2 (aq) + H2 (g)
Okay, so the zinc is reacting with the hydrochloric acid to create zinc chloride and hydrogen gas. The bubbles that are coming out of the reaction are the hydrogen gas. I have tried before to evaporate all of the remaining liquid to gather the zinc chloride compound, but I learned that zinc chloride is deliquescent, which means that it absorbs ambient moisture until it turns itself back into a liquid solution. It is possible to do dry it out, but you have to use high heat, and then store it in a moisture-free environment.
After leaving the penny for 24 hours, retrieve it from the solution with a pair of tweezers (you could use your fingers to take it out, as hydrochloric acid is only about as irritating to the skin as lemon juice, but safety first!). Once placing it in your hand, you will notice how incredibly light and delicate it is. It is basically a really thin copper foil. Rinse it off and leave it to dry, and you now have your own elemental copper! See video below for an example of the bubbling reaction.
Thank you for checking out my experiments, and feel free to comment and ask any questions that you may have. Don't forget to upvote and/or subscribe for future articles!