Pop Your Top (Film Canister Explosions)

How to fuel a film canister rocket with that famous bubbling tablet.

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What happens when you have a buildup of gas? Wait, on second thought, don’t answer that question. The gas in this experiment is nothing more than bubbles of carbon dioxide and the explosion is nothing short of fun.

This experiment requires you to wear protective safety glasses.

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Here's What You'll Need

  • Alka Seltzer tablets
  • Film canister with a snap-on lid
  • Empty paper towel roll (the cardboard tube) or a similar-sized tube
  • Duct tape
  • Construction paper/odds and ends to design a rocket
  • Water
  • Paper towels for cleanup— you already know that this one is going to be good!
  • Watch or timer
  • Notebook
  • Safety glasses
  • **Get everything you need in the Flying Film Canister Kit**

Let's Try It

  1. Put on your safety glasses.

  2. Divide an Alka Seltzer tablet into four equal pieces.

  3. Fill the film canister one-half full with water.

  4. Get ready to time the reaction of Alka Seltzer and water. Place one of the pieces of Alka Seltzer tablet in the film canister. What happens?

  5. Time the reaction and write down the time. How long does the chemical reaction last? In other words, how long does the liquid keep bubbling? Why do you think the liquid stops bubbling? Empty the liquid from the film canister into the trash can.

  6. Repeat the experiment, but this time place the lid on the canister right after you drop in the piece of Alka Seltzer. Remember to start timing the reaction as soon as you drop the piece of Alka Seltzer into the water. Oh, by the way, stand back! If you’re lucky, the lid will pop off and fly into the air at warp speed.

  7. You should have two pieces of Alka Seltzer tablet left. Repeat the experiment using one of the pieces of Alka Seltzer, but this time you decide on the amount of water to put in the film canister. Do you think that will make any difference?

  8. Use the last piece of Alka Seltzer to make up your own experiment. What do you want to find out? How are you going to do it? What are you going to measure?

  9. Go ahead and experiment!

    It’s impossible to do this activity just once. It is addicting and habit-forming. Proceed at your own risk! You’ve been warned.

How Does It Work

The secret is actually hiding in the bubbles that you observed. The fizzing you see when you drop an Alka Seltzer tablet in water is the same sort of fizzing that you see when you mix baking soda and vinegar. If you look at the ingredients of Alka Seltzer, you will find that it contains citric acid and sodium bicarbonate (baking soda). When you drop the tablet in water, the acid and the baking soda react to produce bubbles of carbon dioxide gas.

Carbon dioxide gas builds up so much pressure inside the closed film canister that the lid pops off. The lid is the path of least resistance for the gas pressure building up inside, so it pops off instead of the stronger sides or bottom of the film canister bursting open.

If you tried the experiment again with different temperatures of water, then you also discovered that temperature plays an important part in the reaction. Warm water speeds up the reaction, while colder water takes longer to build up enough pressure to pop off the lid.
We can thank Sir Isaac Newton for what happens next. When the buildup of carbon dioxide gas is too great and the lid pops off, Newton’s Third Law explains why the film canister flies across the room: for every action there is an equal and opposite reaction. The lid goes one way and the film canister shoots out of the tube in the opposite direction.

Take It Further

If you have another Alka Seltzer tablet, divide it into four equal pieces. This time you’re going to determine if changing the temperature of the water has any effect on the speed of the reaction. Repeat the same procedure as before, but change the temperature of the water in each of the four trials and write down your observations. You may need to experiment with several different film canisters until you find one that really pops. It’s important that the film canister has a tight seal or it won’t pop very well.
For a real eye-popping demonstration, fill ten or more film canisters one-half full with water and drop small pieces of Alka Seltzer into each one. Quickly put the lids on the canisters and stand back. Popcorn, anyone?

Alka Seltzer Rocket
Here’s a clever variation of the classic Pop Your Top activity, but this time you launch the bottom of the film canister like a rocket. 3-2-1, blast off!

  1. Start by sealing the end of the cardboard tube with several pieces of duct tape or use a plastic tube with one end sealed.
  2. Divide an Alka Seltzer tablet into four equal pieces.
  3. Fill the film canister one-half full with water. Note: Steps four through six have to take place very quickly or the rocket will blast off before you’re ready. Read the next few steps first to make sure you understand what is going to happen.
  4. Place one of the pieces of Alka Seltzer tablet in the film canister and quickly snap the lid on the container.
  5. Turn the film canister upside down and slide it (lid first) into the tube.
  6. Point the open end of the tube AWAY from yourself and others and wait for the pop. Instead of the lid flying off, the bottom of the film canister shoots out of the tube and flies across the room. Listen carefully and you’ll hear people yelling, “Do it again!”

Once you’ve mastered the technique, it’s time to measure how far the film canister rocket flies across the room. After each trial, write down the amount of water you used in the film canister, the size of the piece of Alka Seltzer (this should not change), and the distance the film canister traveled. What amount of water mixed with a quarter piece of Alka Seltzer tablet produces the best rocket fuel? Hmm . . . sounds like a good science fair project!

After you’ve determined the best amount of water to use, try changing the temperature of the water. How does the temperature affect the speed of the reaction? Does warmer or colder water change the distance that the film canister travels?

If you’re really creative, you can use construction paper to turn the bottom part of the film canister into a rocket. Wrap some paper around the canister, add some fins, top the whole thing off with a nose cone, and you’ve got an Alka Seltzer powered rocket.

Safety Information

This experiment requires you to wear protective safety glasses.

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