HOLLOW PENNIES - Chemists - Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)

Why Is Milk White?: & 200 Other Curious Chemistry Questions (2013)

9. Chemists



Stack of pennies minted after 1982

Metal file, nail file, or emery board

2 teaspoons salt

½ cup vinegar


Thread or wire, long enough to make a necklace

Muriatic acid (optional, available at hardware stores and swimming pool supply houses)

Protective goggles (optional)


In 1982 the US Mint stopped using 95 percent copper for pennies and switched to a cheaper design, made of zinc coated with a thin layer of copper.

In the photo on the right I have a stack of pennies from after 1982, each of which has had a bit of the copper plating filed off the edges. I filed each penny on opposite sides, so the zinc shows through on the top and the bottom edges. If I could remove the zinc from inside, leaving only the paper-thin copper, I could string these hollow penny shells on a string to make a necklace.

As it turns out, you can. Many acids will react with zinc but not with copper. One such acid is hydrochloric acid. You can make a weak (and safe) form by combining about ½ cup of vinegar with about 2 teaspoons of salt (as much as will dissolve in the vinegar). You’ll have to wait a day or two for it to do the trick, though. For a much faster reaction, you can use a stronger form of hydrochloric acid called muriatic acid, available at hardware stores and swimming pool supply houses. When handling muriatic acid, always use eye protection, and make sure you have adult supervision.


Of course, I chose the faster method. Here you can see hydrogen bubbling up from the filed pennies as the exposed zinc reacts with the acid.


The acid also cleans off any tarnish from the copper. As it reacts with the tarnish, more hydrogen is released, and you can see that while most of the bubbles are coming from the filed-off places, there is still some coming from the rest of the penny as the tarnish reacts with the acid.

After a few hours, the pennies will float to the top of the acid and stop bubbling. They float because they are thin copper shells of their former selves and they are filled with hydrogen.


Now you can add a lot of water to the jar to dilute the acid, and it can be safely poured down the sink. Rinse the pennies well, and then you can string them on a thread or a wire to make a necklace.


A close-up view shows the penny is actually hollow.

Many drugs act on the body in similar ways, and when used in combination, they can have harmful effects. Pills that make you drowsy are generally not safe to take when drinking alcohol, for example. The alcohol can increase the effects or side effects of the drug, and the drug can increase the effects of the alcohol.

In other cases, a drug might cause the body to produce an enzyme that breaks down another drug the patient might be taking. Understanding chemistry allows the pharmacist to suggest that the pills be taken at different times or that another drug be substituted for one of the originals.