Water and Air

Blub, Blub; Glug, Glug; Swoosh!

Continuing on in my exploration of how to teach air and water physics with a good balance of “profane” and “reverential” approaches:

For a number of years now, on the first day of teaching water and air physics, after some of the preliminaries of introducing myself and getting to know the students’ names and a little bit about them, I tell them the joke about two young fish swimming in the water who meet an older fish. The older fish says, “Hello youngsters, how’s your water today?” . . . then he swims on. The two young fish look at each other puzzled, and one says to the other: “What the heck is water?”

I took this joke from a famous speech given by David Foster Wallace at a graduation ceremony, and that speech has actually come up more than once in my writing on this Substack in interesting ways. The joke hopefully wakes the kids up a little to the fact that they may have taken the wonder of air and water for granted so far in their young lives.

We then begin with a big “brainstorming” session (nice pun there for air and water physics!) in which we try to think about all the places in the world where water and air are found. The students can always come up with lots of prior knowledge, and this brings to mind that water and air are really everywhere in our world, that they are nearly always found mixed together, and that they are constantly in motion. Water is in our oceans which cover 70% of the planet. The entire planet is covered by a blanket of air that we think of a concentric “spheres” (atmosphere, troposphere, stratosphere, etc etc). Water is found under the ground everywhere in the world. Water is in our air (and air is in our water). Both air and water are in every living thing, including humans and so many human organs have as their activity the movement of air and water smoothly and continuously through the body. We realize through our brainstorming that you don’t just need water; you need to keep water moving through your body to keep healthy . . . and so does every other living thing.

After this brainstorm, the first experiment I do is very simple but profound and instructive. I have a big tub of water (like a big rubbermaid tote. You could do this in your utility sink or bathtub at home), and one or two containers. First, I alway have a narrow-necked plastic water bottle, maybe a half-liter or more in size. The tub of water needs to be deep enough to submerge the bottle completely. Then, it’s nice to also have a wide-mouthed container like a Mason jar. In the school labs where I work, I have access to big graduated cylinders, which are especially satisfying to work with because they have perfectly straight sides and no constriction at the neck. I might also pick a third interesting-shaped container like a narrow neck flask or something like that. I tell the kids that this is a lot like things they might have done playing in the bathtub when they were younger. Then I do the following:

I ask a volunteer to come up and submerge the plastic water bottle without putting the opening underwater. Then I ask them to report on the sensation. The bottle is pushing up, resisting being put into the water. If the student suddenly lets go (which is fun and makes only a minimal water mess, so totally worth it), the bottle “pops” out of the water (and some water pops out too!) with quite a lot of oomph!

Now I ask the volunteer to push the bottle down so the mouth is under the water surface. This creates an air and water exchange that I’ve come to call the “blub-blub”. It’s a very particular sound that’s hard to translate into writing. It’s like (rising in pitch and tempo) . . blub-blu-bl-b-b-b-b-boop! That’s the best I can do, you’ll have to try it yourself to see what I mean, or perhaps you remember the sound from your own playtime in the bathtub!

Now, lift the bottle, full of water, out of the water. It’s heavy now, being filled with water. It is being pulled down whereas, before when it was full of air, it was being pushed up. Now tip the bottle and let it pour out. It’s a similar, but different kind of sound that I call the “glug-glug” sound, and it also changes in pitch and tempo as you continue to pour. Also worth trying and watching and listen to yourself!

We observe that any air-filled container can experience an upward buoyant force if it is pressed down into the water. And any water-filled container held in the air pulls down with the force of its own weight.

When the air-filled bottle is underwater (and actively blub-blubbing), air is escaping due to air’s buoyancy in water, and water is rushing in to fill its place due to water’s weight. When the water-filled bottle is being poured out from above, it’s the reverse. Water is leaving the bottle due to its weight, and air is rushing in to fill the bottle. The students usually notice that the flexible sides of the bottle are alternately caving in and puffing out in rhythm with the blub-blub or glug-glug.

Over the next couple days and with the help of discussion and a few other experiments, we can realize that the blub-blub and glug-glug are demonstrating something fundamental about air and water. The narrow neck allows us to see that pressure differences are causing the air and water to “take turns” coming in and going out. A little bit of air exits the bottle and now the bottle has lowered its pressure inside. Now some water can come in to equalize the pressure. And back and forth and back and forth, like a “pressure pendulum” swinging.

If you do the same thing with a wide-mouth mason jar or a graduated cylinder, you don’t get any blub-blub or glug-glug. Instead you get a “swoosh” as air and water smoothly exchange places with each other. If you turn the container over quickly with the mouth pointing straight down (or cover the mouth with your hand until it’s perfectly vertical, then remove your hand quickly), and you watch quickly and closely, you will see that air will rush up through the center, and water will smoothly fall around the edges, like a waterfall. And, if you invert your water-filled plastic bottle with the narrow neck and give it a little starting spin, you can create a very beautiful vortex that also smoothly flows out, and avoids the glug-glug completely!

I find these experiences to be very rich and that they draw students into curiosity about water and air. . . Since, as you know, I'm not going to prioritize the teaching of materialistic dogma like atoms and molecules, I must work from demonstrations and experiments that invite students into to trying to observe and think for themselves. This means that I need to try to “let the experience speak” rather than telling students what ”the right answer is”.

This simple set of experiences becomes a rich jumping off point into deeper understandings of air and water.  I encourage you to play in the sink or bathtub for yourself!  I'll try to keep sharing with you all along this path of studying air and water as I am able

Phot credit: Me! Glug, glug