Everybody knows adhesion. Adhesion is everywhere. You’ve probably got a roll of adhesive (word derivation FTW!) tape somewhere in/on your desk right now, I’d wager. Cohesion is another story—almost everyone’s seen it in action, but most folks probably don’t realize it. This handy dandy Demo Science science demo will help your smelly students better understand how both ad- and cohesion work. To the Sciencatorium!
The Ol’ Tube-In-A-Tube Trick
For this experiment, you’ll need one medium to large test tube, one small to medium test tube that fits inside the larger one, water, and something in which to catch said water, as you’ll be deliberately spilling it all.
Make sure your test tubes are clean (as an aside, I know some industrial brush manufacturers who can hook you up if you’re in the market for test tube cleaning brushes), and that the fit is a fairly close one. You don’t want them sliding directly against each other, but you don’t want the smaller one bouncing around freely inside the larger one, either. You may want to do some pre-testing to see which size combo works best.
Once you’ve got your test tube size ratio squared away, gather your students (or whoever you’re demonstrating this jazz for) ‘round and fill both tubes with water. Hold them over whatever water catching device you’ve got, and slowly lower the smaller test tube into the larger one. Let go of the smaller tube—it will kind of sink and kind of float, but that’s not the important part here.
Then—just follow me here—invert the larger tube. Turn the whole thing tuckus over teakettle. The water will pour out, obviously, but if everything works out like it should, the smaller tube won’t fall out of the bigger one.* So, did you just become a wizard? No, silly—it’s adhesion and cohesion!
Please Explain to My Brain
As the water dumps out of both test tubes, the smaller tube will kinda sorta “rise” up in the larger one and, ultimately, stick to the inside, glass to glass. This is due to a combo platter of adhesion and cohesion.
Water molecules are polar, and their polarity attracts them to each other, forming beads (or drops) and creating surface tension. This intermolecular force is cohesion, and it occurs when like molecules or materials are attracted to each other.
Adhesion is a similar force that works between different types of molecules. Here, those would be the molecules of water and test tube glass. The combined power of cohesion and adhesion is stronger than that of gravity (in this case, anyway—it certainly wouldn’t work with a Buick in one of those giant shipping containers) and thus prevents the smaller test tube from falling to its doom.
* CYA statement: This is totally on you—Demo Science assumes no responsibility for broken test tubes. That’s the cost of science, baby!