Alka-Seltzer tablets contain citric acid – an acid – and sodium bicarbonate – a base. (And aspirin.) When you drop the tablet in water, the acid and base will mix and react, forming carbon dioxide gas. If this is happening in a tightly sealed container, the gas will expand rapidly, filling the container. This provides propellant for our “rocket” launches.

Film canister rockets – Outdoors Only!

First, you need film canisters. (It’s best to use the style used for Fuji 35 mm film. where the cap fits down into the canister instead of like a Tupperware lid like the Kodak ones do.) If you don’t happen to have a stack of these almost-obsolete items, you can order them from Amazon or elsewhere. (Here’s an affiliate link for film canisters. If you order after using that link, I do get a small referral fee.) I have seen people claim that if you go to the photo department of a drugstore, they may have extra canisters, but I have not tried this.

Break Alka-Seltzer tablets up into quarters. Fill the canister halfway with water. Drop in two quarter-tablets. Seal the canister. Flip it upside down so it’s resting on its lid. Back away. As soon as enough pressure builds, the canister will blow off the lid, launching up to 20 feet in the air with a great popping sound! (This MUST be done outside.)

It takes about 15 – 20 seconds on average. If you want the reaction to be quicker, then after you put on the lid, give the container a little shake before you set it down.

You could also have children test whether they get different results from different amounts of water. Check out this video from Imagination Station, where you’ll see that the more water, the faster it pops. The less water, the higher it pops. (Note: in that same video, they also demo a method for launching 100 alka-seltzer rockets at once!!)

Film canister substitute

If you don’t want to order 15 – 50 film canisters, there is an alternative. There are powdered drink tablets like nuun that come in these canisters. Their lids aren’t as snug as a film canister, so they can’t build up as much pressure, so they only shoot about 3 – 6 feet in the air instead of 20 feet. For maximum height: crumble up the alka-seltzer into small pieces – like an eighth of a tablet. Fill the container only about 1/10th of the way with water. Cap it and shake it, then turn it over.

[Note: in the lids of these canisters, there is a cardboard disk and under that are little beads – my best guess is that these are silica gel?? Before using these canisters for an experiment, you should soak the lid for about 15 minutes and dispose of the cardboard disk and those little beads in the trash.]

Apparently, you could also try making a egg rocket with a plastic easter egg.

Group process notes for teachers:

I can do film canister launches with about 4 kids at once (ages 4 to 6). First, we set a safety line all the kids needs to stand behind when it’s not their turn. Then I call up 4 kids. I hand them each a film canister and a lid, and tell them “just hold these, one in each hand.” Then I fill each halfway with water, and tell them to keep holding them one in each hand. Then I take a film canister from child #1, pop in the tablets, cap, flip, set down – tell that child to get behind the safety line. Then I continue with the next three kids. I do NOT shake the first two canisters, but I do shake the third one a little and the last a lot, so they’ll all go off around the same time. (Note: you could simplify this a little by glueing the alka seltzer onto the lid of the containers, but that wouldn’t be as exciting, I think.)

Math extensions:

Measure how far away the canister lands. (Have a long tape measure! Ours landed 10 – 20 feet from where they were launched.) Count how many seconds it takes for one to pop. Try adjusting variables such as using just one quarter tablet and three quarter tablets, using a little water or a lot, or shaking vs. not shaking, to see how many seconds that takes.

Alka-seltzer substitute

Note: If you don’t have alka-seltzer, try it with a tablespoon of vinegar and 1/2 teaspoon of baking soda: http://ecscienceactivities.blogspot.com/2009/02/kindergarten-chemistry_19.html

Or… mix 1/2 – 1 tsp baking soda with just enough water to make a paste. (Learn more about this method.) Pack it into the inside of a film canister lid.

Fill the canister halfway with vinegar. Put the cap on. Flip it over and step back. (Note: in this video, the set up is done by 30 seconds… the launch is at 1:18 if you want to skip ahead to the excitement.)

Decorating your Rocket

You COULD make little paper rockets to cover the canister with. Make these on card stock or craft foam. (Click here for a printable rocket template to cut out and assemble.)

You shape these rockets around a film canister. You could make it, take the canister out and save it as an art project. OR you can use these paper rockets during a launch to cover the film canister with. It looks cool, but the launch will soak / ruin your paper rocket, so DON’T do it if the child is attached to their art project!!

A Variation for Indoors

If you teach an indoor program, you can do launches indoors – you just need to cover the launch area with a plastic tub. See a video in my Alka-Seltzer Reactions post. In that one, I talk about “exploding paint canisters” but if you don’t want to deal with the paint mess, you can also just do water and alka-seltzer for your covered launches.

A Safety Note

You are working with projectiles here, and the timing is unpredictable. Also, you’re building up pressure till things “explode.” This experiment should be supervised by adults, safety glasses would be smart, and when you’re first testing things out, always start small. Start with a little bit of reactant (e.g. 2 teaspoons of water and a half a tablet of alka-seltzer). If that’s enough, then repeat. If that wasn’t enough to launch things, increase the amounts. Model for the children how to work carefully and step back out of the way of the launch. You’ll note that in one video, the rocket wasn’t launching, so I poked at it a bit with my crutch. I tell children that you should only touch these explosions-waiting-to-happen with a 4 foot pole.

Look here for more fun Chemical Reactions for Kids. And look here for more fun Alka-Seltzer Reactions.