Elephant Toothpaste
Summary
A catalyst makes concentrated hydrogen peroxide dump all its oxygen in seconds, and soap traps that gas into a towering foam. You’ll be able to explain what makes iodide a catalyst1 (not a reactant), why the reaction is so exothermic2, and how an enzyme (catalase in yeast) does the same job differently.
History
The elephant toothpaste demonstration became a fixture of chemistry outreach in the late 20th century, valued for being both instantaneous and unmistakably dramatic. The name comes from the resemblance of the foam column to comically oversized toothpaste — the kind a cartoon elephant might use.
The underlying chemistry is not a novelty. Hydrogen peroxide’s tendency to decompose into water and oxygen had been studied since Louis Jacques Thénard first isolated the compound in 1818. What makes the demonstration work is catalysis: in the absence of a catalyst, dilute hydrogen peroxide degrades over days or weeks. Add a small amount of iodide ion and the reaction becomes hundreds of thousands of times faster, releasing all the oxygen in seconds.
The iodide-catalyzed decomposition was understood mechanistically by the mid-20th century and is now used as a textbook example of a two-step catalytic cycle. The same reaction, scaled up with even higher concentrations of H₂O₂ and manganese dioxide as the catalyst, drives the propellant in some rocket engines and torpedoes.
Hazards & preparation
12% hydrogen peroxide is far stronger than the 3% pharmacy version — moderate hazard.
- PPE: nitrile gloves and eye protection throughout. It bleaches skin and hair and can seriously injure eyes.
- The foam is hot (strongly exothermic) and still oxidizing — don’t touch it for at least a minute. Work on a tray or in a sink and keep spectators back during the pour.
- 12% peroxide is sold as “40 volume” hair developer; 6% is “20 volume.” Both work; 12% is more dramatic.
Disposal: once cool, the spent foam is mostly soapy water — flush down the drain with plenty of water. See the Safety page.
Materials
- Hydrogen peroxide (12%, hair developer) — 100 mL
- Sodium iodide — 5 g (or potassium iodide if available)
- Water — 20 mL
- Dish soap (any liquid dish detergent) — 10 mL
- Food coloring (optional) — a few drops
- A tall, narrow vessel — a 500 mL or 1 L graduated cylinder or narrow bottle works best
- A small cup for mixing the catalyst solution
- Nitrile gloves and eye protection
- A tray or baking sheet to contain overflow
Procedure
Prepare the catalyst solution:
- Dissolve 5 g sodium iodide in 20 mL water in the small cup. Stir until fully dissolved. Set aside.
Prepare the vessel:
- Pour 100 mL of 12% hydrogen peroxide into the tall vessel.
- Add 10 mL dish soap (and food coloring if using). Swirl gently — avoid foaming yet.
The reaction:
- Set the vessel on the tray. Stand back and have observers step back.
- Pour the iodide solution in all at once.
- The foam column emerges within 1–2 seconds and rises rapidly, overflowing a narrow vessel or mounding in a wide one.
For a striped effect: paint three stripes of food coloring up the inside of the dry vessel before adding the peroxide — the rising foam picks up each colour.
What you should see
A second or two after the catalyst hits, a thick column of foam surges up and out of the vessel like toothpaste from a giant tube, steaming faintly and warm to be near. It’s mostly oxygen bubbles wrapped in soap — 100 mL of liquid can throw up litres of foam.
| Symptom | Likely cause | Fix |
|---|---|---|
| Foam small or slow | Peroxide too dilute or degraded | Use fresh 12% (“40 volume”) developer; old bottles weaken |
| Foam collapses at once | Too much / too concentrated soap | Cut to 5–7 mL; avoid “ultra” detergents |
| Just bubbling, no foam | Soap forgotten | Soap must be in the vessel before the catalyst |
| Colours bleed together | Colour added to the liquid, not the wall | Paint stripes on the dry vessel wall first |
| Overflows too fast to see | Vessel too wide | Use a taller, narrower container |
Alternative: Yeast Catalyst
Dry active yeast contains the enzyme catalase, which decomposes hydrogen peroxide biologically. This version is slower and gentler — good for younger children or where sodium iodide isn’t available.
Mix 1 packet (7 g) dry yeast into 50 mL warm water (~40°C), let it hydrate 2–3 minutes, and pour into a vessel set up with 100 mL H₂O₂ and 10 mL soap. With 12% H₂O₂ the foam rises over 5–15 seconds and is shorter and softer; with 3% H₂O₂ it’s a slow, gentle foam safe for very young children. It’s slower because the small amount of catalase throttles itself — the enzyme denatures as the reaction heats up, while iodide has no such limit.
The reactions
The decomposition of hydrogen peroxide is catalyzed by iodide in a two-step cycle:
\[\ce{H2O2 + I- -> IO- + H2O}\]
\[\ce{H2O2 + IO- -> I- + H2O + O2 ^}\]
Net reaction:
\[\ce{2H2O2 ->[\text{I}^-] 2H2O + O2 ^}\]
hydrogen peroxide breaks down into water and oxygen; iodide speeds it enormously and is regenerated, so it’s a true catalyst. The soap traps the oxygen as foam, and the reaction releases heat (ΔH ≈ −98 kJ/mol).
The Science
The rate of uncatalyzed H₂O₂ decomposition at room temperature is negligible — a sealed bottle of 12% H₂O₂ takes months to degrade. The iodide catalyst lowers the activation energy of both steps so dramatically that the same reaction completes in seconds.
The two-step mechanism matters: neither step alone is fast enough, but together they form a cycle where each step regenerates the reagent for the other — a general feature of catalysts.
The foam is a lesson in surface chemistry: soap lowers the surface tension of water so the rapidly evolving oxygen forms stable bubbles instead of escaping. The volume expansion is striking — 100 mL of 12% H₂O₂ releases about 5.6 g of oxygen — roughly 4 litres of gas at room temperature — which with the water vapour driven off by the heat can reach 10–15× the original liquid volume.
Questions to Explore
What makes iodide a catalyst rather than a reactant? It’s consumed in step 1 but regenerated in step 2 — how does the two-step cycle demonstrate the definition?
Hint / answer
A catalyst speeds a reaction and comes out unchanged. Iodide is used up making hypoiodite, then handed back in the second step, so at the end there’s just as much iodide as you started with — it cycled without being consumed. That regeneration is the giveaway of a catalyst.
Why does the reaction need soap to foam? Without it, oxygen just bubbles out. What does soap change?
Hint / answer
Soap lowers water’s surface tension so it can stretch into thin, stable bubble walls that trap the escaping oxygen — turning fizz into foam. Without it the gas can’t hold a film and simply escapes.
Why is the foam hot? It reaches 70–80°C. Where’s the energy from, and why doesn’t a slowly-decomposing bottle heat up?
Hint / answer
Breaking H₂O₂ into water and oxygen releases energy (it’s exothermic); the same total heat comes out whether fast or slow. Done in seconds, that heat has no time to dissipate, so the foam is hot; spread over months in a bottle it leaks away unnoticed.
Why does higher concentration give a bigger result? Same chemistry — does concentration change rate, total gas, or both?
Hint / answer
Both. More concentrated peroxide contains more H₂O₂ per volume, so it yields more total oxygen (bigger foam), and the higher concentration also makes the catalyzed reaction go faster (taller, quicker eruption).
Why does catalase slow down while iodide doesn’t? The yeast version fades as it heats; iodide runs full speed. What happens to the enzyme?
Hint / answer
Catalase is a protein, and the heat it generates makes it unfold (denature) and stop working — it sabotages itself. Iodide is a simple, heat-stable ion, so warming only speeds it up; it keeps going until the peroxide is gone.
Going further
- Compare catalysts. Run the iodide and yeast versions side by side with equal H₂O₂ and time each to peak height — inorganic vs. enzymatic catalysis in one glance.
- Vary concentration. Repeat with 3% and 6% H₂O₂ and see whether foam volume tracks concentration.
- Do the maths. Measure the foam volume and compare it to the oxygen you’d predict from the H₂O₂ used.
Footnotes
Catalyst — A substance that speeds up a reaction without being consumed by it.↩︎
Exothermic — Describing a process that releases heat to its surroundings, making them feel warm.↩︎