Experiment

IMPORTANT: This experiment requires you to wear protective safety glasses.

Pre-Flight Testing

1. Put on those safety glasses.
2. Fill the film canister three-fourths full with soda. To avoid a sticky mess, seltzer water can be used, which is simply carbonated, sugarless water.
3. Quickly seal the canister with the lid and shake the thunder out of the canister! Be careful to aim it away from your eyes. If you're lucky, the lid will pop off and fly into the air at warp speed.
4. What are you waiting for? Do it again! 

The Amazing Alka-Seltzer Rocket 

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 in the film canister into the trash can.
6. Repeat the experiment, but this time place the lid on the container right after you drop in the piece of Alka-Seltzer. Remember to start timing the reaction as soon as you drop the tablet into the water. Stand back! If you're lucky, the lid will pop off and fly into the air at warp speed! Write down your observations.

If you really want to see the rocket fly, start by sealing the end of the cardboard tube with several pieces of duct tape or use a plastic tube with one end sealed. Divide the Alka-Seltzer into four equal pieces. Fill the film canister one-half full with water. Place one of the pieces of Alka-Seltzer tablet in the film canister and quickly snap the lid on the container. Turn the film canister upside down and slide it (lid first) into the tube. 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.

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.

How Does It Work?

The first part of this experiment is just a variation of the classic Alka-Seltzer film canister rocket. The same principle is at work here. In both cases, carbon dioxide gas builds up so much pressure the lid is forcibly launched. With an Alka-Seltzer tablet, the CO₂ is produced as a result of a chemical reaction. With the soda, the CO₂ is produced as a result of vigorous shaking. This provides a good contrast between a physical and chemical change. 

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. The acid mixes with the sodium bicarbonate (baking soda) to produce bubbles of carbon dioxide gas. 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 carbon dioxide gas. The gas keeps building up until finally the top pops off. The lid of the canister 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 canister bursting open.

We can thank Sir Isaac Newton for what happens next. When the build up 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.

Additional Info

Science Fair Connection:

Launching Alka-Seltzer rockets is tons of fun, don't you think? So how could you use this simple and engaging activity for the science fair? The trick is to change a variable, create a new experiment, and then compare the results.

• Repeat the experiment using another of the pieces of Alka-Seltzer, but this time change the amount of water you put in the film canister. 
• Once you've mastered the film canister rocket technique described above, 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 variable), the size of the piece of Alka-Seltzer (this should not change because it is your control), and the distance the canister traveled. What amount of water mixed with a quarter piece of Alka-Seltzer produces the best rocket fuel?
• After you've determined the best amount of water to use, try changing the temperature of the water. How does temperature affect the speed of the reaction? Does warmer or colder water change the distance the film canister travels?

Use another 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? Go ahead and experiment! Just make sure you only change one variable at a time. Document your discoveries and get ready to share them at the science fair.

Observations

You may need to experiment with several different film canisters before you are successful at building a rocket that launches with a blast. If the lid fits too tightly or too loosely, it won't work.