As they’ve almost certainly taped or glued something to something else which the original something should not have been taped or glued to, chances are good that your students know what adhesion is and/or what adhesives are. However, they may not be so familiar with adhesion’s scrappy little brother, cohesion. What is it, and how does it work? The answer to both questions is magic, of course. And by “magic,” I mean science. (This ain’t demomagic.org!) Read on for a handy dandy Demo Science science demo that will help explain cohesion.
Mind the Gap
For this one, you’ll need two glass test tubes, the longer the better; one should be slightly larger around than the other, or, alternatively, one can be slightly smaller around than the other—whichever works best for you. The difference in diameter has to be just right, so maybe try this one a few times to find the right gap size before you do it in class. You’ll also need water and something in which to catch water, like a sink or bucket.
First, make sure your test tubes are hella clean, or the experiment might not work. (Oh yeah, add one of those long-handled industrial brushes to your supply list.) Then, fill both tubes with water, as full as you can fill ‘em.
Hold the tubes over the sink or whatever, and slowly lower the thinner tube into the thicker one. Water will slosh out over the sides of the big one, obviously. As you slide the narrower tube downward, you’ll feel the point at which the water in the bigger tube kind of “takes over” and starts supporting the weight. Push a little past that point, then let go of the small test tube.
Here comes the magic science part: still holding it over your catch basin, invert the larger test tube. SPLASH! SMASH! CRASH! Or not… What happened?
Just Cohesion Doin’ Its Thing
If everything worked out properly, the smaller test tube did not fall out of the larger one and shatter into a thousand pieces on the floor. Instead, some but not all of the water poured of the tubes, and the smaller test tube got sucked up even further into the big one, where it’s held in place by a thin skein of water. This is partially due to adhesion, and partially due to cohesion.
The surface tension betwixt the water’s molecules is a form of cohesion. This intermolecular force occurs when like molecules or materials are attracted to each other scientifically. Adhesion is the attractive force between to dislike molecules or materials. So, the water molecules sticking to one another is cohesion, and the water molecules sticking to the glass of the test tube is adhesion. When combined, these two forces can be even stronger than mighty gravity. Hence, these magical mystery tubes.