Elephants and Chemistry

One of my favorite chemical reactions is frequently called “Elephant’s Toothpaste.”  It creates a LOT of gas from very little liquid, which makes an impresive show, and the main ingredient, hydrogen peroxide, is already familiar to most people. 

Here is series of photos of the Elephant’s Toothpaste reaction (sometimes very fun and very messy go together):

 etp-big-1.jpg      etp-big-2.jpg      etp-big-3.jpg       etp-big-4.jpg

If you ask someone what they use hydrogen peroxide for, they’ll usually tell you it can clean minor cuts and scrapes or that it’s good for household cleaning. 

When you pour hydrogen peroxide on a cut, what do you notice?  The first thing you’ll notice is that it stings a bit, but you’ll also see bubbles at the site of the cut. The bubbles are evidence that there is a chemical reaction; you started with one substance and ended up with something completely different (there was no gas before you poured the peroxide on the cut, and there were gas bubbles after, so the gas is new).

The formula H2O is almost universally understood as another name for water; this chemical formula indicates  two hydrogen atoms stuck onto every oxygen atom. Hydrogen peroxide’s chemical formula is H2O2, so you can think about it as water with extra oxygen attached. When you pour it on your cut, the hydrogen peroxide decomposes into liquid water (H2O) and oxygen gas (O2). The bubbles you see are oxygen bubbles.

In case you were wondering, oxygen atoms don’t like to hang around alone, so they bond to each other and that’s why we get O2 instead of just O.  If you are a person who counts atoms and you noticed that our atoms didn’t quite add up, you’re right; the balanced equation for the reaction is usually written:

2H2O –>  2H2O + O2

The large 2’s mean two of that whole atom (two hydrogen peroxide molecules can react to create two water molecules and one oxygen molecule). If the equation looks strange to you, don’t worry; just know that the molecules do the right thing.

What isn’t included in this equation is that the blood in your cut initiates this reaction (it contains an enzyme called catalase).

Something to try: 

If you have a bottle of 3% hydrogen peroxide at home, I have a project for you (if you are a kid, do this with your parents).

You need 3% hydrogen peroxide, yeast, dish soap, and a cup or bowl.  Most people face bigger hazards in the kitchen every time they cook, but it is a good idea to wear safety glasses or goggles just in case something splashes or falls and breaks, and hydrogen peroxide in your eye would definitely sting.

Put some yeast (I used about a teaspoon of quick-rise yeast) in the bottom of your container:

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Add enough water to wet the yeast and swirl it around or stir it a little:

etp-yeast-and-water.jpg

Now add a little dish soap and swirl or mix again:

etp-soap.jpg

You may want to set the container or a plate to help contain the mess before you add the hydrogen peroxide (I added approximately 1/4 cup (60 mL):

etp-hydrogen-peroxide-1.jpg

etp-hydrogen-peroxide-2.jpg

Here the hydrogen peroxide is starting to react and the soap catches the oxygen gas and it starts to produce foam.

But it keeps going:

etp-hydrogen-peroxide-3.jpg

And going:

etp-hydrogen-peroxide-4.jpg

And going:

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And going:

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The yeast contains catalase (your blood does too), and that helps the reaction happen faster, but the big thing to notice is that a small volume of hydrogen peroxide reacted to create a big volume of oxygen gas (the soap just helped catch it so we could see it better).  Any time you start with a liquid and make a gas, if the gas can expand, it will, and usually a lot.

Oh, and even though the reaction is called Elephant’s Toothpaste, please don’t try to eat it. Yuck!