Activity/Demo Title: Alka-Seltzer Rocket
Demo overview:
The Alka-Seltzer Rocket introduces water to Alka-Seltzer in a sealed container. A chemical reaction occurs and causes bubbling in the form of carbon dioxide gas. The Alka-Seltzer in solid form has far less volume than the CO2 gas, so pressure begins to build in the container. At some critical pressure, the container, if sufficiently small with respect to the Alka-Seltzer and water, will burst. If the container’s lid side is placed down to touch a surface, when the container bursts the lid will remain on the surface and the rest of the container will shoot up like a rocket.
Demo Instructor Notes:
A cheap setup for a rocket consists of an opaque film canister, Alka-Seltzer (or some off-brand), normal room temperature water (we never investigated the effects of water temperature on the rockets, although I imagine hot water would result in a faster reaction), and finally two magnets.
Magnets are critical to this setup, since the Alka-Seltzer tablet needs to only come into contact with the water in the canister when we want it to. We figured out that we can first prop a film canister upright, place a magnet on top of the lid outside the canister, then place a tablet of Alka-Seltzer underneath the lid with a magnet on the other side to keep the tablet suspended (see diagram).
You can then proceed to fill the canister with the desired water. You want enough water to just barely submerge the tablet and cause maximal dissolving, but not so much water that CO2 can’t sufficiently build up. For a film canister, we found that 1/4 gave us rockets that could dent the roof 50’ above.
Now that you have water at the bottom of the canister, and the lid closing the canister with the tablet resting above the water, you can proceed to demo setup. Acquire a home baking tray with high edges to catch water and limit mess. Set the tray upside down and propped up, maybe between two chairs. You should now have a tray for which you can access the “dish” side. Attach the rocket via the outside magnet. Your system should look like the figure below.
When you have attached the desired number of rockets, the last step is to flip over the tray (like the figure below) and wait!
At this point, you have approximately 10 seconds before the rockets go off. You can discuss the conversion of solid tablet to CO2 gas at a much larger volume through a chemical reaction, you can discuss pressure build up and conversion of potential energy to kinetic energy to gravitational potential energy of rocket height.
Some considerations include safety, clean up, and setup quality of life. First, the plastic canister will fall within about 10’ of the tray— they shouldn’t hurt anyone. Second, there will be canisters all over, a wide broom can quickly sweep these out of the picture. Finally, opaque canisters are harder to open than see-through canisters, allowing for more pressure to build up and more explosive rockets. The magnets are hard to remove from the metal tray, so a thin sheet of cardboard can ease the frustration of separating magnets. Depending on your film canister and magnet, you may require more than two magnets— for instance a third magnet between the tablet and lid might be required and would require slightly more water. The tray is extremely precarious; avoid anything that could cause the tray to fall, because even a single drop of water touching a tablet will be enough to set off the rocket. People participating in the show should be made aware of how sensitive the system is.