Floating coins

Three coins floating on water in a bowl.A common classroom demonstration or activity to illustrate surface tension in water is to float a small aluminum (d = 2.70 g/mL) coin such as a Japanese yen in water. Using a similar approach (and a bit more practice before going public) as described in the kit sold by Educational Innovations it is possible to make the demonstration even more dramatic by floating larger aluminum coins such as the pictured French 5 franc (1945) and Mardi Gras “throw” token.

The coin is suspended on a lab tissue and gently lowered onto the water surface where a single drop of food coloring was added to make the demonstration more visible. At this point students are told that they should not be impressed by the floating; the tissue might play a major role in the behavior. It is then possible to gently push the edge of tissue under the water working around the edges of the tissue until it sinks completely. Now the coin alone is floating.

The French coin weighs 3.569 g with a diameter of 31.07 mm and thickness of 2.05 mm. The Mardi Gras “throw” token, so named because it is thrown from the New Orleans parade floats, weighs 4.627 g with a diameter of 39.01 mm and thickness of 2.18 mm. The yen weighs only 0.992 g with a diameter of 20.00 mm and a thickness of 1.46 mm. It takes a bit more practice to float the larger coins but it is worth it. (I do not recommend that you try the demonstration after three quick cups of coffee!) It is not the comparative calculations that makes this demonstration memorable to students, but the visual impact of the large floating coins. When viewed from the side, the two larger coins are actually below the extended surface of the water. Among the advantages of using the larger coins is that the surface curvature around each coin is more visible.

When students are asked for an explanation of how and why the coin is floating, they invariably know the correct multiple choice answer “surface tension”. However, they are much less clear on what surface tension is or what its molecular origins might be. Some are actually surprised that liquids other than water also exhibit surface tension.

It is instructive to do quick calculations to show that floating the larger coins is not trivial. Floating the franc is actually harder than floating the larger Mardi Gras token. Although heavier than the other two coins, the token is relatively thin and actually easier to float than the franc; simple calculations make that readily apparent. The mass of each coin is distributed over different areas:  for the yen, the franc, and the token the mass/area is 0.316, 0.471, and 0.387 g/cm2, respectively. The pressure (in Pascals) exerted on the surface is also readily calculated. For these coins it is 31.0, 46.2, and 38.0 Pa for the yen, franc, and token, respectively.

P = (m x g)/(π x r2)

in units of kg, m/s2, and m2 for mass, acceleration due to gravity, and coin area, respectively.

These calculations were based on the actual measurements for the coins and not the nominal descriptions that appear in catalogues. Other coins may yield slightly different values. (Because of the engraving and the raised rim on each coin, calculating the density from the size measurements is only moderately accurate. However, each item is listed as unalloyed aluminum in coin and catalogue references.)

With no special equipment or supplies needed, this is an activity that students may be given as a challenge to practice anywhere, in the classroom or at home. Once floating, the coins are surprisingly stable and I have left the demonstration on my desk for days at a time for students to see during office visits. By carefully replenishing the water as it evaporates, the system may be maintained for days, if not weeks.

[If you want to win some floating coins from Educational Innovations, enter our monthly contest — Milky Way su-chem-du on page 15.

For the winning cover photo, Leon has won a copy of “Mad Scientist” written and donated by Theodore Gray.]