Launch an Alka-Seltzer Rocket

What can make a beautiful spring afternoon even better? Rockets of course! In this activity we will learn about chemical reactions and propulsion by launching Alka-Seltzer rockets. Back when I worked as a science camp instructor, this was my favourite activity.

What you'll need:

  • Film canister (like this)
  • Alka-Seltzer tablets (like this)
  • Water
  • Decorating materials

First, decorate your film canister. I decided to decorate three canisters to resemble some of the robots in If: Ball, Then: Catch. My advice would be not to get too attached to your creations... more on this later.

This activity will involve spraying water, so head outside and get ready to launch your rockets! You'll want to find a space that is wide-open because your rockets can go pretty high up in the air.

A quick note on safety: while these are pretty small, they are rockets. This activity should be performed with adult supervision. Make sure that no one is close to your rocket when it is being launched, and make sure it is pointing straight up in the air (not towards anyone!).

To launch your rocket, first pour water into the film canister until it is about one-third to one-half full.

The next step will need to be done quickly so make sure you are standing at the launch site. Add about a quarter of an Alka-Seltzer tablet to the water; notice that it will start to fizz. Put the cap onto the bottom of the film canister, flip it over, and place it on the ground. Now stand back!

For a few seconds it will look like nothing is happening, but be patient! You should see your rocket blast-off within a minute. If it has been more than about three minutes there may be a leak in the canister. Wait a few more minutes to be extra sure, then carefully check if the canister is empty. Wipe off the edge of the lid and canister, and then try again.

Three, two, one, blast off!

As I mentioned before, don't get too attached to your decorations! My Tin Can robot lost his wheels on his first launch.

So what is going on here? When you mix the Alka-Seltzer and water you get a chemical reaction. The Alka-Seltzer tablet contains a few different chemicals, but the two that we care about are called citric acid and sodium bicarbonate. When these chemicals get wet they react together to make carbon dioxide gas bubbles. This is the same gas that you make when you breath out.

Normally these gas bubbles would just pop and move out into the air, but when you trap them inside of a film canister they have no where to go. This starts to build up pressure inside the canister. Eventually there is so much pressure that the lid will pop off!

Newton's Third Law of Motion says that "for every action, there is an equal and opposite reaction." This means that when you place your rocket lid-side-down, the gas rushing out of the bottom of the rocket pushes the rest of your rocket way up into the sky. This is called propulsion!

How high does your rocket go? Try experimenting:

  • More or less water
  • More or less Alka-Seltzer
  • Different rocket shapes
  • Different materials for your launch pad

Have fun!

This activity is based on the book If: Ball, Then: Catch, available here!


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