Density: Ivory Soap vs. "Other" Soap

Another simple demonstration involving density....

Show the students a bar of Ivory soap and a bar of any other brand of soap.  Ask for predictions about what will happen when the bars are placed in a tub of water.  Float or sink?  Will the both do the same thing, or will they do different things? 

Then put the bars in the water, using as much flair as you deem necessary.

The Ivory soap will float, but the other soap will sink. 

Of course, now the work begins and it's time to hypothesize why the difference.

I'm sure your students will come up with all sorts of possibilities (the more the better), but the reality is that Ivory whips more air into their soap a they're making it.  More air pockets = lower density. 

You could have the students calculate the density of each of the bars.  If you're using nice rectangular bars, the volume can be calculated using dimensional measurements.  If you're using a funky shaped bar of soap, as this bar of Zest is, you'll need to use water displacement to get the volume.