It looks like a giant squeezed toothpaste out of the bottle. The foam shoots up, rolls over the sides, and keeps coming for several seconds. The trick: a tiny bit of yeast turns ordinary drugstore peroxide into a gas factory, and dish soap traps the gas as foam.
What you need
- An empty 500 ml plastic bottle with a narrow neck (an old water bottle works well)
- 100 ml of 3% hydrogen peroxide — the kind sold at the drugstore as a wound rinse
- 1 tablespoon of active dry yeast
- 3 tablespoons of warm water (not hot — hot water kills the yeast)
- 1 tablespoon of dish soap
- Food colouring
- A large tray, baking sheet, or sink to catch the overflow
Safety note: Only use the 3% drugstore kind of hydrogen peroxide. Stronger peroxide (salon-grade, lab-grade) can burn skin and eyes — do not substitute. Wear safety glasses. Have an adult help. Never put the lid on the bottle after mixing — the gas builds pressure fast. Keep your face back from the bottle opening. The foam is mostly soap and water once the reaction is done, but don’t rub it into your eyes — wash hands afterward and rinse skin with water if any splashes on. If any gets in your eyes, rinse with clean running water right away and tell an adult. Don’t drink any of it. The reaction overflows fast, so the tray is not optional. The bottle gets warm; let it sit a minute before picking it up.
Steps
- Stand the empty bottle on the tray.
- In a small cup, stir the 1 tablespoon of yeast into 3 tablespoons of warm water. Wait one minute — the yeast wakes up.
- Pour the 100 ml of peroxide into the bottle, then add the 1 tablespoon of dish soap and a few drops of food colouring. Swirl gently.
- Pour the yeast-water into the bottle all at once.
- Step back and watch.
Tip: A narrower neck makes the foam shoot up taller. A wider mouth makes it spread.
What’s happening?
Hydrogen peroxide is water with one extra oxygen atom — its formula is H₂O₂, not H₂O. That extra oxygen makes the molecule unstable. Left alone, peroxide slowly falls apart on its own into water and oxygen gas:
2 H₂O₂ → 2 H₂O + O₂
But “slowly” means weeks or months. Yeast contains an enzyme called catalase that grabs peroxide molecules and rips them apart in a fraction of a second. That’s a catalyst — a substance that speeds up a reaction without being used up itself. The yeast is still yeast at the end.
Catalase is also in your blood and most of your cells. It’s the reason hydrogen peroxide fizzes when you put it on a cut: your own cells are doing the same thing the yeast is doing here.
The dish soap doesn’t take part in the chemistry — it just traps the oxygen gas as bubbles, which is why you get a tall column of foam instead of an invisible puff of gas. Touch the bottle: it’s warm. The reaction releases heat, so it’s exothermic.
Key vocabulary
- Decomposition reaction — one substance breaking apart into two or more simpler ones
- Catalyst — speeds up a reaction without being used up; can be reused
- Enzyme — a catalyst made by a living thing; catalase is an enzyme
- Exothermic — releases heat; the bottle gets warm to prove it
- Oxygen (O₂) — the gas you breathe in; here, the gas blowing the bubbles
Math connections
You poured 100 ml of 3% peroxide into the bottle. ”3%” means 3 parts in every hundred — so 3 grams of actual peroxide is mixed into enough water to fill 100 ml. The solution is mostly water, so a millilitre weighs about a gram.
- How many grams of peroxide is in 100 ml? → take 3% of 100 ml of solution = 3 g
Bonus: how much gas comes out?
For older readers — the rest of the lesson works without this part.
Chemists count molecules in big groups called moles. A mole is a counting word like “dozen” — only enormously bigger. The handy fact: one mole of any gas takes up about 24.5 litres at room temperature, no matter what gas it is.
The chemistry equation says two peroxide molecules make one oxygen molecule, and one mole of peroxide weighs about 34 grams. So divide your peroxide grams by 34 to count moles of peroxide, halve it (because two peroxide molecules make one oxygen molecule), then multiply by 24.5:
- Oxygen gas produced ≈ 1081 ml (just over 1 litre)
So 100 ml of liquid turns into about 1081 ml of gas:
- Expansion ratio: 1081 ÷ 100 ≈ 11×
That’s why the foam is so dramatic — every spoonful of liquid becomes around 11 spoonfuls of bubbles. Compare with Lesson 2: Vinegar Volcano — same idea (the gas does the show, not the liquid), different gas (CO₂ instead of O₂).
Scale it up: if you doubled the recipe to fill a 1000 ml bottle, you’d need 200 ml of peroxide. (Double the yeast, water, and soap too — and double the size of the tray.)
Try more
- No-yeast control jar: pour the same peroxide-soap mix into a second jar with no yeast. Watch it for an hour. Almost nothing happens. That’s the catalyst’s job, made visible.
- Striped foam: squirt two different food colours down opposite walls of the bottle before pouring the yeast in. The foam comes out striped.
- Bottle shape: repeat the experiment with a wide-mouth jar instead. Same amount of foam, but it spreads instead of shooting up.