Q: Where does the “elephant toothpaste” foam come from?
I’m not sure where the name “elephant toothpaste” came from. Some kid probably thought the foam looked like toothpaste coming out of a tube, and since the toothpaste is so big it must be for an elephant.
I can tell you where the foam comes from. It doesn’t contain the same ingredients as regular toothpaste. In elephant toothpaste, concentrated hydrogen peroxide is mixed with liquid soap and combined with a catalyst to create an exothermic reaction.
Hydrogen peroxide (H2O2) is a chemical compound that can be used as a bleaching agent or disinfectant. Over time, it breaks down into water and oxygen:
2 H2O2 (aq) ⟶ 2 H2O (l) + O2 (g)
(The abbreviation “aq” stands for “aqueous.” This means that the substance has been dissolved in water. The other abbreviations used are for liquid (l) and gas (g).)
If you mix hydrogen peroxide solution with soap, you end up with a soapy mixture. The oxygen produced from the decomposition tries to escape, pushing through the solution. The soapy solution traps the oxygen, creating bubbles. Capturing a lot of oxygen in a lot of bubbles makes foam.
The problem is that hydrogen peroxide decomposes slowly on its own- too slowly to make a visually appealing demonstration. However, the decomposition process can be sped up using a catalyst. A catalyst lowers the activation energy, the energy required to start a reaction. This makes it “easier” for a reaction to start, and the rate goes up.
The catalyst used in elephant toothpaste is often potassium iodide– or more specifically, the iodide ion in potassium iodide. Below is the catalyzed reaction.
H2O2 (aq) + I– (aq) ⟶ H2O (l) + IO– (aq)
H2O2 (aq) + IO– (aq) ⟶ H2O (l) + O2 (g) + I– (aq)
You might notice that we start with iodide and end with iodide. Catalysts are not consumed, so once a catalyst is used in the reaction, it can be used again.
This entire process is exothermic– it releases heat. If you were to hold your hand above the foam, you would actually feel just how hot it gets.
Food coloring added before the catalyst can give the foam a certain color. If no coloring is added, the foam will be a light yellow due to the presence of iodide.
You can find a recipe for the above “lab version” here. There is also a “home version” here, which uses yeast instead of potassium iodide and a lower concentration of hydrogen peroxide. If you perform this demonstration, be sure to use safety precautions.
Reminder: this shouldn’t be used as toothpaste for humans… or elephants.
Keep calm and science on.