Were to ask them, which maybe you should as a good setup for this experiment, most of your students would probably say that rocks are hard and solid, among other things. Most rocks are pretty hard, no denying that, but they’re not always so solid. There are copious types of porous stone out there, and this Demo Science science demo will help you explain porosity to the smelly little kids you occupy your classroom day in and day out.
You’re on your own in explaining to them where the “u” from “porous” goes when it becomes “porosity”, though.
Add Fire to See All Four Elements in Action
For this little experiment, you’ll need an assortment of porous rocks (think igneous—pumice, scoria, basalt, etc.), water, and something in which you can combine the two. For simplicity’s sake, we’ll assume you have a sink (with the drain plugged) handy.
Fill your sink with several inches of water, enough to cover the rocks by a good few inches. Then gather your students (or whoever you’re doing this demonstration for) around and hand out the rocks to the kids of your choosing. DO NOT give Kevin a rock—you just know he’s going to lick it or worse.
Have the kids give the stones a good once-over, and ask them which one they think has the most air in it. There’s a good chance they’ll look at you funny, because “there’s no air in rocks”. Ah, the stupidity of youth!
After they’ve made their predictions (or non-predictions, as the case may be), have your rock holders place the rocks in the water. After but a moment—lo and behold!—air bubbles will be flowing out of the rocks. In your skeptical little faces, students!
Science Rocks (Get It? “Rocks”)
All the science in this one stems from our good friend porosity. The more porous the rock in the water—the more spaces between the minerals that make up the rock itself—the more air bubbles you’ll see. Some of your porous stones will cause a lot more action than others. And, depending on their weight and the collective force of the tiny air bubbles pouring out, the rocks may move slightly, entirely on their own.