If you have taught as long as I have, you have lived through at least two really annoying disposable water bottle fads. Bottle flipping. Everywhere, everyday. The constant drone of bottles being flipped and then the obligatory , OOOOOOHs when someone successfully did it.
Then came the lid launching phase. Empty the bottle, twist it up tight, and launch the plastic cap into your friend’s eye hole. Then you leave the lid wherever it lands so the teacher picks it up later.
Both of these were pretty annoying. Makes you wonder what will come next. But the lid launching actually has some benefits to your classroom. The gas laws!
All the kids are familiar with lid launching, but now through simple demo, you can connect the combined gas law to it. We get to redeem an annoying fad as well as making the gas laws feel very familiar. That’s a win.
Lid Launching Demo
In case you haven’t partaken of the lid launching fad, simply empty a disposable water bottle. Put the lid back on. Start twisting the bottle until you can decrease the volume by about half. This builds up a lot of pressure inside. Then, while keeping the pressure high, use your thumb to twist off the cap. The cap will launch with some seriously velocity, leaving a water vapor trail streaming from the bottle.
Pro-tip: squeeze a little air out of the empty bottle before twisting so you can really increase the pressure.
Pro-tip: loosen the lid ever so slightly before launching so that you don't look like a weakling in front of the class trying to twist a lid off with your thumb.
Boyle’s Law
Pressure and volume are inversely proportional. This the most easily seen law in the lid launching demo. As you twist up the bottle, you are reducing the volume. Therefore, the pressure inside the bottle increases. They already know this intuitively, but now you can put a name to it. You can also reference how the bottle maintains a constant number of moles of gas.
Charles's Law
Absolute temperature and volume are directly proportional. This law is harder to see in the lid launching demo. You can take your sealed water bottle and put it in ice water to show the volume decrease somewhat. However, I combined the lid launching demo with the can crushing demo. If you have never done can crushing, you really should. Simply take an empty pop can and put a few mL of water in the bottom. Set it on the hotplate until it is boiling. Have a tub of ice water nearby. Once the water in the can is boiling, use tongs to quickly invert the can into the ice water. This lowers the temperature rapidly as well as lowering the volume of the gas. The can crushes violently. Kids love it. Warning: If you do this demo for your own children when they are young, they will ask to crush every empty can in the house for the next several months. So, strap in.
Gay-Lussac’s Law
Pressure and temperature are directly proportional. This one might not be obvious to students. But they have, in fact, already noted this while shooting lids. They just didn’t know it. After launching the lid, there is often a little plume of “smoke” that comes out of the bottle. Of course it isn’t smoke, and you will have to tell your kids that. It is condensed water vapor. Why would water vapor condense all of a sudden? Because temperature drops. As pressure drops, so does average kinetic energy. This leads to a rapid cooling of the gas which causes the intermolecular forces to take over. In water’s case, hydrogen bonding is sufficient to make little water droplets. It is “smoking” because it is cold. It is cold because the pressure went from very high to very low very quickly. Since pressure and temperature are directly proportional, the temperature also dropped.
Combined Gas Law
Combined gas law is just the synthesis of the three laws above. When kept at constant moles, the three equations form one. By doing this demo, I actually lead kids straight to the combined gas law. I teach proportions from it, and not from the individual laws. If pressure, volume, or temperature stays constant, then you can just ignore it in the combined gas law, leaving one of the other laws. Kids seem to find this easier to handle. But since I have done the lid launching demo, they have a connection to all of the individual laws.
Using the lid launching demo is an easy way to connect the student’s real-world experiences to the gas laws. Plus, if the lid happens to launch into the eye hole of “that one student”, at least it was done in the name of science. But you didn’t hear that here.
Tired of kids feeling lost when you introduce moles? Try Up & Atom. This game allows students to start comparing numbers of atoms among atoms, moles, and grams all the while they are playing a fun strategic game. Check it out.
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