Friday, February 12, 2016

The Catalytic Decomposition of Hydrogen Peroxide--Bust and Boom!

Please don't call it elephant's toothpaste.

The kids have been studying atoms and how they combine into molecules, and exploring chemical reactions is an excellent way to expand upon and enrich that study. Chemical reactions take molecules and make them into different molecules or break them down into atoms, and you can see that microscopic process with your very own eyes!

Science is amazing, isn't it?

One of the most accessible chemical reactions with a great wow factor is the catalytic decomposition of hydrogen peroxide. It's become common to refer to this demonstration as "elephant's toothpaste," and I have to admit that I just don't get that. Yes, it's a cute name. No, it doesn't relate to anything about the process or the science behind it. Yes, kids like cute names. No, it doesn't do kids any good to expose them to the demonstration without any explanation, as if it's simply a fun magic trick. SCIENCE is the magic here, my Friends! Let your kids know that this is SCIENCE!

*steps down off soapbox*

Ahem... anyway, there are several different ways to force the decomposition of hydrogen peroxide, but as we learned for ourselves, some are less user-friendly than others.

We first tried forcing the decomposition of hydrogen peroxide using yeast as a catalyst:



It was a total bust. I doubt that it worked any more quickly than simply exposing the hydrogen peroxide to the sun would. We played around with it, had to leave for an activity, came back hours later, and were like, "Oh, it finally foamed up... yay."

Another day, we tried the demonstration again with proper supplies, and it worked like a dream.

Instead of jacking around with household supplies, I bought the actual proper ingredients, both 20-volume hydrogen peroxide and 40-volume hydrogen peroxide (for an interesting comparison) and potassium iodide.

I bought the powdered form of potassium iodide, because it's simple to make it into a saturated solution--just 1 gram of potassium iodide powder in 1 ml of water, shaken well:

You also need some sort of wide-bodied container with a small opening. When we tried this demonstration outside, we used a plastic soda bottle, hoping for an explosive exit by the oxygen, but on this day, the temperature was in the teens, and I instead chose an Erlenmeyer flask for our indoors demo. 

For the first demonstration, I poured 1/8 cup of hydrogen peroxide into the Erlenmeyer flask, then added the saturated solution of potassium iodide. The result is not, like, amazingly exciting, but does give you a clear view of the oxygen atoms being released from the hydrogen peroxide molecules:


Don't forget to touch the outside of the flask while you're observing the chemical reaction. This is an exothermic reaction, so you'll feel the warmth! 

The chemical reaction becomes more exciting when you add something for the oxygen molecules to cling to as they escape. A big squirt of dishwashing detergent should do nicely:



One of the especially fun things about this demonstration, as I'm sure you can see, is that the byproduct is entirely heat, oxygen, and bubbles. It's perfectly safe to play with:




This is the face that a kid should make when doing science!

This is also a great time to review the structure of molecules. We use this Zometools Molecular Mania kit:

With it, the kids modeled both hydrogen peroxide and water. See that extra oxygen atom in the hydrogen peroxide model? That's what we were playing with!

Finally, I asked the children to repeat the demonstration on their own. Here's Will making her saturated solution of potassium iodide:

Success!

This was one seriously fun demonstration, and I can't imagine a better way to make that information on atoms, molecules, and chemical reactions stick.

Just wait until we get to do this again outside! And yes, when I promised the kids that we could make it into a "bomb," I meant it.

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