Elephant Toothpaste
Difficulty: Easy | Time: 10 minutes | Visual Impact: Extremely High
Historical Context
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. Concentrated hydrogen peroxide degrades faster, but still slowly enough to be safe in a bottle. 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 propulsion of the propellant in some rocket engines and torpedoes.
Safety
12% hydrogen peroxide is significantly stronger than the 3% pharmacy version. It can bleach skin and hair on contact and cause serious eye injury. Wear nitrile gloves and eye protection throughout. Do not touch the foam immediately after the reaction — it is hot (the reaction is strongly exothermic) and still oxidizing. Work on a tray or in a sink. Keep children back from the vessel during the pour.
12% hydrogen peroxide is sold as “40 volume” or “20 volume” hair developer at beauty supply stores. Check the concentration on the label — 12% is 40 volume; 6% is 20 volume. Both work, but 12% gives a much more dramatic result.
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 in order, working quickly once the catalyst is ready.
Prepare the catalyst solution:
- Dissolve 5 g sodium iodide in 20 mL of 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 of dish soap. If using food coloring, add it now. Swirl gently to mix — avoid creating too much foam at this stage.
The reaction:
- Position the vessel on the tray. Stand back slightly and have observers step back.
- Pour the iodide solution into the vessel all at once.
- The foam column will emerge within 1–2 seconds and rise rapidly. With a narrow vessel it will overflow the top; with a wider vessel it forms a broad mound.
The foam is hot — do not touch it for at least a minute. After it cools, it is safe to handle; it is mostly soapy water and can go down the drain.
For a striped effect: Before adding the hydrogen peroxide, run three stripes of different food coloring up the inside of the vessel with a thin brush or dropper. The rising foam picks up each color as it passes.
Alternative: Yeast Catalyst
Dry active yeast contains the enzyme catalase, which decomposes hydrogen peroxide through a biological mechanism. This version is slower and gentler — suitable for younger children and classrooms where sodium iodide is not available.
Additional materials: 1 packet (7 g) dry active yeast, 50 mL warm water (~40°C)
Procedure: Mix the yeast into the warm water and let it hydrate for 2–3 minutes — a small amount of fizzing confirms it is active. Set up the vessel with 100 mL H₂O₂ and 10 mL dish soap as in the main procedure. Pour the yeast slurry in all at once.
What to expect: With 12% H₂O₂ the foam rises over 5–15 seconds rather than 1–2, and the column is typically shorter and softer. The reaction is still noticeably warm. With 3% H₂O₂ the yeast version produces a slow, gentle foam — safe for very young children and needing no protective equipment beyond caution.
Why it is slower: Iodide catalyzes the reaction chemically; catalase does it enzymatically. Catalase is faster per molecule than iodide but the amount of enzyme in a yeast packet is small, and the enzyme denatures (unfolds and stops working) as the temperature rises during the reaction — it throttles itself. Iodide has no such limit.
Comparison experiment: Run the iodide and yeast versions side-by-side with identical H₂O₂ amounts. The difference in speed, foam height, and heat generated directly illustrates the difference between inorganic and enzymatic catalysis.
Reactions
The decomposition of hydrogen peroxide is catalyzed by iodide in a two-step cycle. In the first step, iodide is oxidized to hypoiodite:
\[\ce{H2O2 + I- -> IO- + H2O}\]
In the second step, hypoiodite oxidizes another molecule of hydrogen peroxide, regenerating the iodide and releasing oxygen gas:
\[\ce{H2O2 + IO- -> I- + H2O + O2 ^}\]
Net reaction:
\[\ce{2H2O2 ->[\text{I}^-] 2H2O + O2 ^}\]
The iodide is fully regenerated — it is a true catalyst, not a reactant. The soap traps the oxygen as it erupts, creating the foam. The reaction is strongly exothermic (ΔH ≈ −98 kJ/mol), which is why the foam is hot.
Yeast variant — catalase mechanism:
Catalase is an enzyme whose active site contains an iron(III) heme group. It catalyzes the same net decomposition in two steps. In the first, one molecule of H₂O₂ oxidizes the iron center to a high-valent oxoferryl intermediate called Compound I:
\[\ce{Catalase\text{-}Fe^{III} + H2O2 -> Compound\ I\text{-}Fe^{IV}{=}O + H2O}\]
In the second, a second H₂O₂ molecule acts as a reducing agent, regenerating the resting enzyme and releasing oxygen:
\[\ce{Compound\ I\text{-}Fe^{IV}{=}O + H2O2 -> Catalase\text{-}Fe^{III} + H2O + O2 ^}\]
Net reaction — identical to the iodide route:
\[\ce{2H2O2 ->[\text{catalase}] 2H2O + O2 ^}\]
The same oxygen is liberated, the same foam forms, and the same heat is released. The difference is purely in how the activation energy is lowered: iodide does it through a simple two-step redox cycle in solution; catalase does it by binding and orienting the substrate inside a protein pocket, stabilizing the transition state through precisely positioned amino acid residues. Catalase is one of the fastest enzymes known — each molecule can decompose tens of millions of H₂O₂ molecules per second under ideal conditions — but the small amount present in a yeast packet, combined with thermal denaturation as the reaction heats up, limits the observable rate.
The Science
The rate of uncatalyzed H₂O₂ decomposition at room temperature is negligible — a sealed bottle of 12% H₂O₂ takes months to degrade appreciably. 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. This is a general feature of many industrial catalysts.
The foam structure itself is a lesson in surface chemistry. The soap reduces the surface tension of water so that the rapidly evolving oxygen forms stable bubbles rather than escaping immediately. The same principle underlies fire-suppression foam. Without soap, the oxygen simply bubbles out and no foam forms.
The volume expansion is striking: 100 mL of 12% H₂O₂ contains about 4 g of dissolved oxygen — enough to produce roughly 3 liters of gas at room temperature. Combined with the water vapor driven off by the heat, the foam can reach 10–15× the volume of the original liquid.
Troubleshooting
| Symptom | Most likely cause | Fix |
|---|---|---|
| Foam is small or slow | H₂O₂ is too dilute or degraded | Use fresh 12% developer; old bottles lose potency. Check the label — “20 volume” is only 6% |
| Foam collapses immediately | Too much soap, or soap is concentrated | Reduce to 5–7 mL; avoid “ultra” concentrated detergents |
| No foam, just bubbling | Forgot the soap | Soap must be in the vessel before the catalyst is added |
| Foam is warm but not hot | Concentration is below 12% | Use fresh 40-volume developer |
| Colored stripes bleed together | Food coloring added to the liquid rather than the vessel wall | Apply coloring to the dry wall of the vessel before adding H₂O₂ |
| Vessel overflows too fast to see | Vessel is too wide | Use a taller, narrower container (graduated cylinder, wine bottle) |
Explore Further
Compare catalysts: Run the iodide and yeast versions (see above) simultaneously. Time from pour to peak foam height. Measure the foam volume and the temperature of each. Which catalyst is faster? Which produces more heat per unit of foam?
Vary concentration: Try the same procedure with 3% H₂O₂ and 6% H₂O₂. Is the foam volume roughly proportional to the H₂O₂ concentration?
Measure the temperature: Push a thermometer into the foam immediately after the reaction. How hot does it get? Calculate the expected temperature rise from the enthalpy of decomposition and compare.
Catalyst recovery: After the foam subsides and the liquid cools, the iodide is still present. Add a fresh portion of soap and another 100 mL of 12% H₂O₂ directly to the spent liquid. Does the reaction repeat at the same rate?
Volume calculation: Measure the volume of foam produced. Given that 12% H₂O₂ contains 12 g of H₂O₂ per 100 mL, calculate the theoretical volume of O₂ released at room temperature and pressure. How closely does the measured foam volume match?