Short answer: A fizzy, fizzy geyser — and it9’s mostly physics, not chemistry
Drop a roll of Mentos into a bottle of Coke and you get a spectacular, sudden fountain of foam. It9s not because the candy is magically reacting with the soda; it9s because the soda is full of dissolved carbon dioxide and the Mentos give the gas a thousand tiny places to form bubbles at once. The result is rapid bubble formation, foam, and a dramatic ejection of liquid from the bottle.
What exactly is happening?
Supersaturated soda and the eager gas hiding inside
Soda is bottled under pressure. That pressure forces a lot of carbon dioxide (CO2) into the liquid so the drink can hold more gas than it would at normal atmospheric pressure. When you open the bottle, some CO2 escapes slowly as bubbles, but a lot of it remains dissolved — the liquid is supersaturated with gas.
Nucleation: the spark that turns fizz into a geyser
For CO2 to form a bubble, it needs a starting point. That starting point is a nucleation site: a tiny imperfection, a pit, a speck of dust, or a rough surface where gas can gather and grow. Mentos are full of those microscopic pits across their surface. When dozens of Mentos hit the soda at once, they present thousands of nucleation sites in a very small time window, and CO2 forms bubbles everywhere simultaneously.
Surface tension and how the candy helps the bubbles grow
Beyond surface roughness, ingredients on the Mentos9 surface (and some ingredients in the soda) lower the surface tension of the liquid. Lower surface tension makes it easier for tiny bubbles to expand instead of collapsing. The combination of tons of nucleation sites plus reduced surface tension sends bubble growth into overdrive and pushes foam out of the bottle.
Is it a chemical reaction?
No — not in the way vinegar and baking soda react. That classic kitchen volcano is an acid-base reaction that forms new chemicals and releases CO2. The Coke-and-Mentos show is physical: it’s a release of dissolved gas driven by nucleation and surface effects. If you9re curious about true chemical fizzing, try the vinegar-and-baking-soda experiment — it9s the same fizzy thrill but with chemistry doing the work.
Why Diet Coke often makes the biggest fountains
If you9ve watched videos, you9ve probably seen Diet Coke used more than regular Coke. Why? Several factors make diet sodas especially dramatic:
- They contain artificial sweeteners (like aspartame) and other additives that change surface tension in ways that favor big foam.
- Without sugar syrup to make the foam sticky and heavy, the bubbles stay light and rise quickly.
- Diet varieties are typically less viscous than sugary sodas, so foam can form and shoot out more freely.
That said, sugar is not a strict blocker — you can get impressive fountains with regular Coke too, but the textures and behaviour may differ.
What variables change how big the geyser gets?
People love tweaking this experiment because one small change can make a huge difference. Here are the main levers:
- Temperature: Warmer soda holds less dissolved CO2, so at higher temperatures the gas wants out more quickly. Warm bottles often give bigger, faster eruptions (but also fizzier, less controlled results).
- Carbonation level: A bottle shaken, freshly opened, or intentionally over-carbonated contains more free CO2 and can produce stronger fountains.
- Number of Mentos: More candy = more nucleation sites = more bubbles. A whole roll makes a bigger show than a single mint.
- Drop speed & simultaneity: Dropping all Mentos at once gives the sudden, concentrated burst that produces a tall geyser. One-by-one drops create smaller, staggered effects.
- Type of candy: Mint Mentos, fruit Mentos, or other candies have different surfaces and coatings. Fresh, dry Mentos with a rough surface tend to perform best.
- Bottle size & opening: Larger bottles can feed taller fountains; wider openings let foam escape faster, sometimes reducing height but increasing flow volume.
How to try the experiment safely (and a few tips for better results)
Materials
- 1 or 2 liter bottle of soda (Diet Coke is a popular choice)
- 1 roll of Mentos or a handful (start with 5–10 for a smaller test)
- Mentos dropper or a simple paper tube to drop them all at once (optional but helpful)
- Outdoor space, goggles or safety glasses, and clothes you don9t mind getting sticky
Procedure
- Take the experiment outside and place the bottle on flat ground.
- Open the bottle and, if you want more drama, insert a tube or dropper that holds the Mentos above the opening.
- Quickly release the Mentos into the soda and step back.
- Observe how the foam shoots out and measure or record the height if you9re being scientific.
Safety notes
This is a spectacle, not a hazard: the reaction doesn9t create harmful chemicals or heat. But the spray is messy and can be slippery, and getting sticky soda in eyes or on electronics is unpleasant. Do it outdoors, wear eye protection if you9re close, keep cameras and people away from the bottle mouth, and be ready to rinse off sticky surfaces.
Fun variations and experiments to try
- Compare different sodas (regular vs diet, cola vs fruity) and record which makes the tallest fountain.
- Test temperature: run trials at cold, room, and warm temps to see how height changes.
- Try other candies or objects with rough surfaces to see whether they act like Mentos (most won9t be as dramatic).
- Time the eruption: how long until the flow stops? Does more Mentos change the duration?
- Use different numbers of Mentos and plot Mentos vs fountain height — you9ll see a steep early rise that levels off.
How this compares to other fizzy experiments
There9s a neat contrast between the Mentos trick and classic chemical fizzes. For example, the dramatic bubbles in a vinegar-and-baking-soda reaction are produced by an acid-base chemical reaction that actually forms new molecules while releasing CO2. The Mentos-and-soda geyser doesn9t change the chemistry of the soda; it just lets trapped gas escape quickly. If you9re interested in hands-on fizz that involves chemical change, the vinegar-and-baking-soda post is a great next stop.
Why this trick still delights scientists and kids alike
Part of the charm is how simple and surprising it is. You look at a Mentos and a Coke every day and think of them as snack items, not components of a dramatic physics demo. The trick folds tactile texture, dissolved gas, and fluid dynamics into a single, theatrical moment. It9s a gateway to real scientific thinking: form a hypothesis, run a test, measure results, and revise your idea.
Quick recap: the three ingredients for a great Coke-and-Mentos show
- A supersaturated, carbonated drink (lots of CO2).
- Many nucleation sites introduced quickly (the Mentos surface).
- Low surface tension and low viscosity so bubbles grow and escape rapidly.
Want to keep exploring?
If you like the physical drama of the Mentos geyser but also enjoy chemistry-based fizz, try a controlled acid-base experiment like vinegar and baking soda to see how the mechanisms differ. Or, if you9re curious about kitchen chemistry more generally, there9s a whole shelf of household pairings that reveal different scientific principles — some depend on chemical change, others on physical properties.
Science is full of little surprises like this: ordinary things behaving in extraordinary ways when you give them a fresh context. Try a careful experiment, make a messy mess, and then tell me what strange, wonderful result you found.