Ask Mad Physics: Alka-Seltzer Bottle Rockets
Question: I am trying to demonstrate the effects of expansion and pressure to a group of younger students and I’d rather stay away from doing anything too dangerous. Is there a good (and inexpensive) experiment I can do for them or have them try?
Answer: I did this experiment for a group of 5-8 year olds in Australia and they loved it. It’s very simple, inexpensive, but still packs a decent pop.
Principles | Alka-Seltzer | Demonstration
The main concept here is pressure. Pressure refers to the magnitude of the normal force over a certain area. This means that even if the same force is applied to two different objects, the resulting pressures will most likely be different.
Take for example a nail. This is am everyday item that one uses around the house. It requires little force to drive it through drywall; however, consider a different situation. Imagine punching a wall! You could apply just as much force onto the wall as you had with the hammer, but I doubt your first punctured the wall. Why is this?
Well although the amount of force (F) remained constant the surface areas that made contact with the wall (the nail and the hand, respectively) were not equal. Since your hand has a bigger surface area than the tip of a nail the force was (roughly) evenly distributed, and thus the amount of force per unit area (A) was much lower than the amount of force per unit area on a nail. So, the best mental picture to have for this situation is to consider that a smaller surface area will be able to concentrate energy more efficiently than a bigger one.
Therefore, from this one could deduce the equation for pressure (P):
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This lab demonstrates the principles above by launching a small film canister through the expansion of gasses and thus by creating pressure. It should be noted that although the materials involved in this lab are rather trivial, you should be careful what type of film canister you use. The ubiquitous black and grey cases found in the United States are prone to having weak lids and thus generally let the pressure escape with no bang! The translucent canisters are preferable. Since we had none handy we bought a similar container from the container store.
For our propellant we used Alka-Seltzer. Alka-Seltzer is a water-soluble tablet owned by the Bayer Corporation. It is used to treat headaches and indigestion. It is made up of aspirin and baking powder, and since baking powder fizzes in water, we can trap the gases in a container and use the stored energy to launch the film canister. To do this experiment one should fill a canister halfway with water and then drop a tablet inside. The cap should be immediately replaced and the bottle should be flipped to rest on its lid. Once the pressure increases enough the body will pop off leaving only the lid on the ground.
This experiment is both messy and slightly dangerous (don’t poke an eye out) so take care.
If you want a visual guide to how this experiment is set up click here for a quick video.
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Family, Afrooz. “Alka-Seltzer Bottle Rockets.” 17 Oct. 2005 Mad Physics. dd mmm. yyyy †
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