It’s May, and that means that we teachers need lessons with some extra oomph to compete against sunny blue skies and the ice-cream truck’s siren song. (It's right outside my classroom window, I kid you not!) The “Exploding Soap Experiment” is a perfect three-day project that teaches a lot of math and science, while your students beg for more sudsy fun. After all, kids love anything billed as an EXPLOSION!
This project is great for students who have an understanding of the concept of mathematical volume. So, before diving into this one, make sure your students have had plenty of tangible experiences with volume (submerging items into water and calculating the displaced water volume, as well as with centimeter cubes) and have built a secure understanding of the relationship between area and volume for rectangular prisms.
A student explores volume by building structures with centimeter cubes.
Assuming your students have an understanding of the "length x width x height" method for finding cubic volume, then this experiment is for them! The students need to be adept at calculating volume to collect experimental data, so I begin by having them practice measuring and calculating areas. I borrow a selection of wooden blocks in many sizes from a kindergarten colleague and label the blocks alphabetically using sticky notes. Then my students measure the blocks and calculate the volume. They need to accurately find the volume of at least three blocks to receive their “ticket” to participate in the experiment. (I provide one-on-one support for students who are tripped up by volume. Of course everyone eventually earns a ticket.)
A student measures a wooden play block to calculate the volume using L x W x H = V.
It also helps if your students already know about density as the relationship between mass and volume. If not, you may first want to detour into buoyancy and density activities (some of which I’ll blog about in the future). Your students will be calculating the density of various brands of soap to predict which brands will float or sink. To do this, they need to find the volume of their bars of soap, as well as the mass. Then they divide the mass by the volume and — voila! — they have the density!
Measuring bars of soap with centimeter rulers to calculate the volume.
Another student weighs his soap in order to calculate the soap's density.
My students know that objects with a density greater than 1g/cm3 will most likely sink in water, which helps them with their predictions. We also conjecture about possible relationships between density and how the soap will “behave” in the microwave. I show them a small sample of “exploded” soap to start the conversation, but I don’t tell them which brand it is! I let them know that some brands of soap will not explode (puff up), some will melt, and others will barely show a physical change.
You may also want to share the “Legend of the Floating Ivory® Soap” with your students. According to soap lore, in the late 1800s an Ivory soap factory worker left the mixing machine running while he took his lunch break. So much air was incorporated into the soap that the first floating soap was “invented,” and it quickly became a favorite product of bath-takers everywhere. Turns out that this story is more fiction than fact, but regardless, Ivory soap does have a higher than usual air content, and thus it floats. (It’s not the only floating soap nowadays, though!)
Spoiler alert: If you want to be surprised about which soaps “explode,” don’t read on. Go test a sampling of soaps. I really had too much fun peering into my microwave “preparing” for this lesson. As you probably guessed, Ivory soap definitely puffs up to great heights in the microwave. But many other brands of soap perform in interesting ways when microwaved for a minute. Please be careful handling the hot soap! Microwave soap on paper plates, use oven mitts, and allow the soap to cool for a few minutes before students touch the soap.
Be warned that this is also a very perfumed experiment! If you don’t want your classroom to have a rather overpowering soapy scent, you might want to microwave the soap near a window or even outdoors. I used an extension cord to power our microwave in the schoolyard.
After exploding the bars of soap, you can store the altered soap in resealable plastic bags near the sink for your students to use. Hand washing became a favorite activity when manipulated soap was available. Be sure to leave time for students to chart the results of the microwaving test and draw conclusions about density and soap-exploding behavior.
To prepare for this project, I asked my students to each bring in a bar or two of soap so I could amass a wide variety. Make sure you have the classic Ivory soap on hand, since this is a guaranteed exploder.
I cut the bars of soap into thirds using a knife and cutting board. Then I stored the prepped soap in labeled baggies. Students will pick six varieties to measure and test. To find the volume/mass/density you’ll need centimeter rulers, balances, and calculators. You’ll also need a “test tank” for the floating/sinking step. I use a clear plastic storage bin. And you’ll obviously need a microwave, which will smell soapy for several days.
We use a water-filled bin as our float/sink test tank. Soaps are organized into clearly labeled cups.