Elephants Toothpaste (Jacob Drori)

Title: Elephants' Toothpaste

Principle(s) Investigated: Chemical vs Physical reactions, Catalysts, Decomposition

Standards : MS-PS1-2. MS-PS1-4.

Materials: Hydrogen Peroxide (3% and 30%) Potassium Iodide, Yeast, dish soap

Procedure:

• First: have one Erlenmeyer flask for each student or group. It doesn't really matter the size so long as you scale up everything else. I'm going to assume you have a 250ml flask.
• Second: Fill 1 test tube with 1 active yeast packet (About 1/2 tbsp), 1 test tube with dish soap (about half way) , 1 test tube with warm water (about 1.5 tbsp), and a few test tubes with the 3% hydrogen peroxide (equalling about 1/4 cup in total). (I'm using 18mm x 100mm tubes, but feel free to scale and mess around).
• Third: Give the students goggles. Safety is a priority of course.
• Fourth: Have them mix the water and the yeast in the Erlenmeyer flask and swish it around for about 30 seconds.
• Fifth: Add the soap. More swishing.
• Sixth: Add all the hydrogen peroxide in one shot.
• Seventh: Write down what you observed!

I've also attached two sheets from the University of Utah and North Carolina State University's respective chemistry departments outlining how to do a much more volatile version of this experiment which I will be demonstrating. This one involves slightly more dangerous versions of hydrogen peroxide and should probably only be done by the instructor as a demo, rather than a hands on activity.

Student prior knowledge: The difference between chemical and physical reactions. The basic composition of hydrogen peroxide.

Explanation: First, you have to mix hydrogen peroxide with dish soap. This reaction causes the release of a lot of gas, so if you add dish soap it captures it, allowing the creation of quite a bit of foam, which is more exciting than just making lots of vapor. Then you need to add a catalyst, in this case yeast or potassium iodide, which makes the hydrogen peroxide break down very quickly. Hydrogen peroxide is H₂O₂ and decomposes into into oxygen and water. It will do so on its own slowly, depending on its temperature, but these catalysts cause it to break down at an incredibly accelerated rate. It's worth noting that if you want to test the hydrogen peroxides breakdown levels at various temperatures, it has a nasty habit of exploding when boiled.

In this experiment, we use two types of hydrogen peroxide. The first is a standard 3% solution found over the counter. It's incredibly diluted, and as such perfectly safe for your kids to use in class. They've probably put it on cuts before and watched it foam up. As a fact worth mentioning, it doesn't actually do much for cuts once they've started healing, and can even slow the healing process down.

The other hydrogen peroxide was at around 30-35% concentration. This kind you can usually get at beauty stores since it is a part of hair bleaching kits. It is difficult to get anything more concentrated than this since hydrogen peroxide is an incredibly powerful oxidizer and quite dangerous.

Yeast and potassium iodide both work to help accelerate the breakdown of hydrogen peroxide into oxygen and water. The chemical reaction in general is

2H₂O₂ → 2H₂O_(l) + O_2(g)

If you want to also look at the iodide ion in the reaction, it looks like this

This reaction is exothermic, or heat producing. The kids can touch their foam and feel that it is slightly warm. The large demonstration, however, produces very hot foam and isn't advised to be touched.

Questions & Answers:

1) Given the properties of hydrogen peroxide you've seen (specifically its breakdown and release of oxygen) what's one potential use you could think of beyond what we typically see?

Answers may vary, but it is used occasionally as a rocket propellant.

2) Why do you think hydrogen peroxide is used so much as a disinfectant?

Hydrogen peroxide in high concentrations is excellent at sterilizing surfaces and is a safe alternative to chlorine bleach since it breaks down into oxygen gas and water. In low concentrations like we see in drug stores, it can kill some bacteria on the surface, but it also kills off the new cells that are growing to try to heal the area, and as such can lead to slower healing and scaring. Part of the reason I suspect it to still be used is due to the foam it makes, which gives people the idea that, "it's working" it sure is, but not always for the best.

3) Can you think of any animal besides humans that would use Hydrogen peroxide in any way?

The bombardier beetle uses it as one of its defense mechanisms. It mixes it with hydroquinones in its body, creating a violently exothermic reaction and spraying the mixture at a temperature of about 100 C.

Applications to Everyday Life: Can you think of any real world application for chemical reactions that release oxygen or other gasses? How about ones using yeast specifically?

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: