The porosity and permeability of the various materials that make up the soil on any given place on Earth can play a huge role in the flow of groundwater and surface runoff. So, if any of your students like rivers, lakes, or other bodies of water, they should at least know a little bit about this stuff, eh? You’re dang skippy they should!
To help you ensmarten the smelly little punks, Demo Science offers this handy dandy science demo.
When Is A Full Jar Not Full?
For starters, you’ll need a large jar (an empty pickle jar works great—go for what I call “Moby Dills”, ‘cause the bigger the better [accurate for both pickles and jars]), a bucket of dime-sized pebbles (enough to fill your jar), and water.
First, fill up your jar with pebbles. Be careful when pouring the pebbles into the jar, because a broken giant glass jar underneath a pile of tiny rocks is a heckuva thing to clean up in front of a mess of smelly kids.
After you’ve filled the jar without breaking anything, ask your students if the jar is full. Many, if not most, of your students will probably say that yes, it is in fact full. The kids that know that it isn’t really full are going places—concentrate most of your teaching efforts on them for the rest of the year. The rest are lost causes.
Then, follow up questions: “If it’s full, will I [you] be able to add any water? If so, how much?” Take bets if you’d like—it’s an easy way to make about a dollar in change and score some half-eaten gummies. Slowly add water to the jar while your students observe the water’s movement through it.
Optional Follow Up Experiment: Repeat the same process, but with sand in the jar instead of pebbles.
Optional Follow Up Follow Up Experiment: Wait a while, say an hour or so, and pour more water into the sand jar. Ask students how much more water they think you’ll be able to add. Repeat after an even longer interval—overnight, perhaps.
When It’s Not Full, That’s When!
In the first—and/or only, depending on how ambitious you’re feeling—part of this experiment, students will easily be able to see how the water moves through the pebbles to fill the empty spaces (or voids) between them. Point out to them that larger materials provide better porosity—that is, essentially, greater amounts of empty space between them.
As the water fills the rest of the space in the jar, it is an excellent visual example of permeability. Permeability, of course, is the ability of a material to transmit fluids. (Of course!) The secondary experiment, with the sand, shows how different permeability levels affect the flow of water, as the water will take much longer to flow into the sand than it did the pebbles.