Have you ever placed Ivory soap in the microwave? Whewwww-weee!! The kids love it!!
I start with 4-5 different brands of bar soap. As a class, we fill out a sink/float prediction and investigation sheet to document which brands of soap float and which don't.
We realize that Ivory soap is the only brand of bar soap that floats, and I guide them to wonder why. Students will sometimes ask if there are air pockets, so I usually cut the bars of soap in half and let them look at the soap from the inside. They see that, while Ivory soap doesn't have air pockets, perse, it does look different than the other bars of soap. It has a slightly "whipped" look. So I give them some background information:
Ivory soap was the first bar of soap to ever float, but it was made that was on accident. One of the Ivory soap workers left his stirring machine on too long, and it ended up whipping the soap and putting more air into it. Folks liked that the bar of soap would float when they dropped it in the bathtub, so the Ivory soap company continued to make their soap that way.*
*That's a story I read on the Internet before doing this investigation with my students. Feel free to double-check your facts with a little Google search of your own. :)
We talk about what other things might have air in them that would float. Specifically, I guide students to think about popcorn kernels and marshmallows. This leads to a comment about how, when heated, marshmallows and popcorn kernels puff up, sometimes popping. This leads us to wonder what would happen if we heated Ivory soap. The students are so surprised when I say, "Hmm. Let's do it."
I puff up 1 bar of Ivory soap per 8 students (since my lab groups are in groups of 4, I just smoosh two tables together for a giant group of 8) and let them play in the soap for a few minutes before cleaning up.
We practice conservation by saving the leftover soap flakes and using them to wash our hands for the next month or two.
Word of caution #1: the soap flakes get EVERYwhere! I wouldn't do this activity if you have a carpeted classroom, for sure. Not only do they get everywhere on the floor, but they also float up into students' noses and mouths. I definitely recommend blowing a large fan to keep the circulation going. Your room is going to smell soapy clean when this is done!
Word of caution #2: if you plan on using that microwave for food again, be prepared to scrub. It's best to clean the leftover soap-goo off your microwave walls and tray immediately, but sometimes it's either a) too hot to touch! or b) an inconvenient time because you're still teaching! Since I teach 4-5 classes a day, I stupidly left my soap-goo to dry until the end of the day, and THEN tackled it to clean. I scrubbed for 10-15 minutes before it all came off.
It's messy, but TOTALLY worth it. The kids will talk about this investigation for weeks!