Chemistry: Elephant Toothpaste (Kid-Friendly Version)
[Shopping List: ~10% hydrogen peroxide solution ; yeast; food colors; dish soap; small graduated cylinders; a tub or other container to control the mess]
- Fill the graduated cylinder about 1/3 full with the hydrogen peroxide solution, add a few drops of liquid dish soap and one or two drops of food coloring.
- In a separate container, mix one packet of active dry yeast powder (rapid-rising types work best) with about half a cup of warm water, stir and let sit for ~ 5 minutes.
- When you are ready, pour some of the yeast mixture into the graduated cylinder (until it's about 2/3 full) and watch the reaction!
What's Happening: Hydrogen peroxide (H2O2) decomposes into water and oxygen gas, but normally the reaction is so slow as to be imperceptible. What happens when you pour hydrogen peroxide onto a cut? It bubbles! That's because there is something in your bodily fluids that catalyzes the decomposition. A catalyst is a substance that speeds up a reaction, without being consumed itself.
In this experiment we use a 10% hydrogen peroxide solution (much more concentrated than the 3% solution from drug stores). The production of oxygen gas is made more noticeable by adding some dish soap, which traps much of the gas and makes a foam. The reaction is catalyzed by the active yeast added to the container. The yeast changes the mechanism, or pathway, by which the reaction occurs. The rapid production of bubbles of oxygen gas, along with the dish soap, quickly creates a large quantity of foam, as well as a lot of heat. This experiment provides another opportunity to discuss solids, liquids, gasses, as well as the other in-between states like foams and gels.
[For our demonstrations we typically use 15-30% hydrogen peroxide for a much more dramatic reaction, but not safe enough for kids to try. In the past we have done a more kid-friendly experiment using 6% peroxide from drugstores, but this is becoming very hard to find. We then tried 10-15% peroxide from beauty supply stores (sold as 30-40 Volume "Developer"), but the problem with this is that manufacturers usually add stabilizers to the solution to extend its shelf life, i.e. to prevent it from decomposing too quickly, exactly what we want it to do in this experiment. So even though the hydrogen peroxide concentration is higher, the net result is about the same if not even a little worse sometimes than using drug store 3% solution. Thus the best option is to buy concentrated hydrogen peroxide (30%) which requires extreme care, and dilute it to make a 10% solution which still requires adult supervision, but is a good compromise between safety and an impressive reaction.]
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