Extra yardage out of the floating penny

Penny floating on liquid.I do demos not only to spice up my Honors Chemistry classes but to help my students make connections between the macroscopic and microscopic worlds of chemistry and ultimately, to help them achieve a deeper understanding of a principle. The problem is that my students view demos, first and foremost, as entertainment; in fact, oftentimes they get so caught up in the wow-factor that they miss the educational connections I’m trying to make.

For some demos, I bait my students to think deeply about the chemistry behind the show with the lure of extra credit. I’m not one to allow students to bump their grades just by writing a report or doing some other form of busy work. But, every now and then, I offer a few points for thinking beyond the expectations of my course. It always amazes me how hard students will work for bonus points, especially in comparison to their effort for regular points.

One of the demos I embellish with extra credit is the floating penny. I do this demo sometime after we’ve discussed the composition of pennies and after we’ve studied single replacement reactions, often in the middle of winter when we need a pick-me-up. All the set-up occurs the afternoon before I want to use it in class and takes about five minutes (honestly!). I place about 75 mL of 6 M HCl* in a 250 mL beaker and place the beaker somewhere in the classroom where it’s unlikely to be disturbed yet some place where the whole class can easily observe it. Then, I take a post-1983 US penny — or a post-1996 Canadian cent — and score the copper shell with a triangular file in 3-4 places evenly spaced around its circumference so that the zinc core is visible. I drop the penny in the acid and go back to my office.

Overnight, the acid oxidizes the zinc — but not the copper — and tiny bubbles of hydrogen gas are produced that can be seen slowly escaping through the notches in the copper shell.

Zn(s)  +  2HCl(aq)  →  ZnCl2(aq)  +  H2(g)

Much of the gas gets stuck in the copper shell and eventually causes the seemingly normal penny to float to the surface. While the demo is super easy to do, sometimes the gas leaks out of the copper shell so the penny doesn’t float. To make sure I’ve got at least one floating penny to show off in class, I set up 2-3 identical beakers each containing a penny.

I set aside about 15 minutes, usually at the end of the period, for the activity. First, I tell the students the rules: Soon they will see a cool chemistry “trick” and they can earn up to 5 extra credit points toward the next test if they can explain how it works. They can work alone or in groups of up to three students and must submit their explanation in writing by the end of class. They may use their notes and their books, but they may not use the Internet.

Penny floating on liquid from the side.Then I reveal the beaker and the floating penny. I tell them that the liquid is hydrochloric acid; that I dropped the penny in the acid the night before; that it initially sank and that it was floating by the morning. I don’t tell them what I did to the penny or that the penny has a zinc core. Those with keen observation skills will notice the notches. The groups soon spread out, some to pore over their books while others linger around the beaker. Those that watch the penny often notice various clues, the most important being the bubbles escaping from the notches in the copper shell. I don’t answer too many questions, but I do respond to those that could be answered with an experiment such as, “Would this trick work in pure water?” or “Would both an old and a new penny float in the acid?”

It’s rare that I get many explanations that warrant the full 5 points. But that’s not the point; extra credit is only the bait to get them to think hard. And, in my experience, they take not only the bait but the hook, line and sinker as well!

Editor’s note

We encourage chemistry teachers to share their extra yardage demo suggestions with Chem 13 News readers. Email Jean Hein, jhein@uwaterloo.ca. We will be featuring Jean Weaver’s extra yardage demonstrations in the next few issues of Chem 13 News and would love to see this as a regular feature.

*Look up the SDS information for 6 M HCl. Keep in mind good ventilation is required given the volatility of 6 M HCl. HCl is corrosive so preventing spills is important. I used to do this demo in a graduated cylinder, clamped in place. I have switched to beakers because they are more stable. It is important that no one (i.e., students) comes in and tampers with the acid or pokes at the penny. I recommend securing the room and/or placing a warning sign near the beaker — especially since it needs to sit out for several hours.