The “Elephant Toothpaste” experiment is a very popular, albeit messy chemistry demonstration. To carry out this experiment, place a 250 mL graduated cylinder on something that you wouldn’t mind getting messy. Next, add 75 – 100 mL of 30% hydrogen peroxide (H2O2, CAUTION: Wear goggles and gloves. 30% H2O2 can cause burns!). Once this is done, add 10 – 20 mL of Dawn (or similar) detergent. When you are ready to run the reaction, quickly pour 10 – 20 mL of concentrated, aqueous potassium hydroxide (KI) into the peroxide/detergent mixture and stand back! The potassium iodide catalyzes the decomposition of hydrogen peroxide, producing a lot of oxygen gas:
2 H2O2 (aq) --> 2 H2O (l) + O2 (g) (Equation 1)
The released oxygen gas produces an enormous amount of foam as it diffuses through the detergent. The amount of foam produced is surprisingly large, and the reaction is quite exothermic. I enjoy running this reaction in class, and my students enjoy it, too! If the experiment is repeated using 3% hydrogen peroxide in place of 30% hydrogen peroxide, students can observe the effect of reactant concentration on reaction rate.
When performing this reaction, I almost always observe yellow-brown streaks in the foam, along with a pungent odor. Both of these observations are consistent with the production of some iodine (I2). If I2 is indeed produced, then at least some of the iodide (I-) must be acting as a reactant, not a catalyst in the reaction:
2 H+ (aq) + H2O2 (aq) + 2 I- (aq) --> I2 (aq) + 2 H2O (l) (Equation 2)
Notice that the oxygen gas (and therefore the foam) produced in this reaction results from the reaction indicated in Equation 1, but not Equation 2. A fun exercise is to get students to suggest ways to increase foam production by maximizing the process whereby I- acts as a catalyst (Equation 1) and minimizing the reaction where I- is a reactant (Equation 2).
However, my favorite twist on this experiment is to demonstrate that there really is oxygen gas is in the foam produced. This is done by lighting a wooden splint, subsequently blowing it out so the splint barely glows, and then plunging the glowing splint into the oxygenated foam. Because of the high concentration of oxygen trapped in the foam, the splint re-lights and even burns quite well in the foam. Sometimes, the foam lights up like a light bulb! My students especially appreciate it if I act like Professor Snape in “Potions Class” and yell “LUMOS!” just prior to plunging the glowing splint into the oxygenated foam. This is a great experiment to do around Halloween. Check it out in the video below:
Have you ever run the Elephant Toothpaste reaction in your class? Have you tried any of the modification suggested here or any others? Have you observed the production of iodine in this experiment? How do you maximize foam production in this experiment? I would love to hear from you, so please comment with some new twists for me to try with this fun and fascinating reaction!