Alka-seltzer tablets contain citric acid (an acid) and sodium bicarbonate (a base.) When you drop the tablet in water, they mix and react, forming carbon dioxide gas which bubbles up to the surface of the water. If the water and alka-seltzer are in a tightly sealed container, the gas will expand rapidly, which can lead to some fun experiments.
Fun with Bubbles
Pour oil, water, and food coloring into a jar or water bottle.Then add an alka seltzer tablet. Here’s how Steve Spangler describes the science of what happens: “the Alka-Seltzer tablet reacts with the water to make tiny bubbles of carbon dioxide. These bubbles attach themselves to the blobs of colored water and cause them to float to the surface. When the bubbles pop, the color blobs sink back to the bottom of the bottle.” It’s infinitely re-useable. Any time you want more bubbles, just add another Alka-Seltzer. If you don’t have Alka-Seltzer, Hands On As We Grow says salt works too.
In my Chemical Reactions post, I illustrate how to mix vinegar and baking soda in a bottle, then put a balloon over the top of the bottle – the gas from the chemical reaction inflates the balloon. You could do this with water and an Alka-seltzer tablet in the bottle.
Chemical Reactions Demo: Put 2 – 3 tablespoons of water and a tablet of alka-seltzer in a zip-lock baggie. Seal it – ask kids what they see – bubbles. Walk around, holding it up to kid’s ears – what do they hear? Fizz. Ask them to notice what’s happening to the baggie – it swells. Feel the bag – it’s cold. Endothermic reaction.
Explosion: Ask the kids what might happen if you put more tablets in a baggie of water. Test it. (Put in the tablet, seal the bag, set it inside a big plastic tub.) This is similar to the ziplock time bombs I describe in the Chemical Reactions post.
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 sitting around, you can easily order them from Amazon or elsewhere.
Prep: Break Alka-Seltzer tablets up into quarters.
In class: 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. (Or test whether you get different results from different amounts of water. In this video, I notice that the one with the most water popped quickest. And the one with the least water popped highest. Note: in that same video, they also demo a method for launching 100 alka-seltzer rockets at once!!)
Group process: I can do this with about 4 kids at once. 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, or shaking vs. not shaking, to see how many seconds that takes.
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
If you don’t have a film canister, try making a egg rocket with a plastic easter egg and a toilet paper tube.
Exploding paint bombs
Take film canisters. Fill them about halfway with a water-based paint (we used 1 part tempera mixed with 2 parts water). Add half of an alka-seltzer tablet. Quickly seal. Shake a bit. Set down upside down on a piece of paper. Quickly move away! http://kidsactivitiesblog.com/75017/exploding-paint-bombs-activity; www.housingaforest.com/exploding-art/
Note: this could make a very big mess. Since we’ve done this in a public park and inside a building, we wanted to contain our mess more, so we were glad to come across this YouTube video www.youtube.com/watch?v=bfNpNTXuWMc which demonstrates how to contain this experiment inside a plastic tub. It’s also lots of fun because the containers go off gradually over a long and unpredictable period of time, and each makes the pop when it launches, then a loud THUNK when it hits the top of the tub! We did this one as a demo, not hands-on for the kids.
Prep: Glue a half an alka-seltzer to the inside of each lid. (Thanks to https://www.imaginationstationtoledo.org/educator/activities/rockets for this idea, which simplifies the process.) Put paper down on the floor. Line up the prepared paint canisters on the paper, with their lids next to them, and a big plastic tub next to them.
Show time: Put all the lids on. Turn all the containers over, shaking if desired, then put them lid down on the paper. Quickly! Put a tub over them. Wait. (Expect about 25 seconds to first pop.)
Pop a Cork
You could use a test tube with a stopper or a glass bottle with a tight-fitting cork. Fill with either alka-seltzer and water, or vinegar/baking soda. Test to see what quantity you need to pop the cork. http://childcentralstation.com/2011/08/perfecting-blowing-the-cork.html.