Experiment with Pumice
In this science experiment, kids explore the properties of pumice vs. other rocks.
Pumice is a combination of minerals, so scientifically speaking, it is a rock. (A rock, by definition, is the combination of two or more minerals.) Pumice is referred to as an “igneous” or “volcanic rock; ” it is formed when lava explodes out of a volcano as a light froth full of gas bubbles, then cools very quickly. The gas bubbles leave holes in the rock, making it extremely light – usually light enough to float. Pumice is often used to make lightweight concrete blocks; it is also used in landscaping, and to exfoliate rough skin.
Fill a small plastic tub with water. Experiment with adding various rocks that are not pumice. What happens? (Most, if not all, should sink.) Now try putting the pumice in the water. What happens? Why do you think the pumice floats?
Be sure to examine the pumice with a magnifying glass to see where the gas bubbles left holes when the lava cooled into a solid.
You can find pumice in most drug stores with the hand and nail care products.
If you have access to other volcanic rocks, such as obsidian or basalt, it would be interesting to compare them to pumice. How are they the same? How are they different?