Our group wrapped up another year of science investigation with the now-traditional Explorers finale: launching streams of soda pop high into the air.
This popular—and messy, so do NOT do this inside!—activity is done by many but understood by relatively few. If you can manage to get some of the small round candies known as “Mentos” quickly down into a full 2-liter bottle of pop, a rather explosive reaction takes place: the liquid comes shooting up and out of the bottle, spraying anyone in nearby proximity.
The question the Explorers were required to contemplate before being allowed to play was a simple one: WHY does this happen? After the squirting has died down the Mentos are still present at the bottom of the bottle—a little smaller from some dissolving, perhaps, but clearly they themselves didn’t simply explode. So what powers the pop eruption?
The answer lies in a relatively obscure (from a 5th grade standpoint, anyhow) science term: “nucleation.” Matter—which is what everything that has mass and takes up space is made of—comes in three main forms or “phases”: gases, liquids, and solids. Which phase any particular type of matter is in generally depends on its temperature: to change a solid into a liquid, or a liquid into a gas, you need to raise the temperature, and to go the other way you need to lower the temperature. For example, raising the temperature will melt an ice cube into a puddle of water, and raising it even more will change it into steam, a gas; on the other hand, lowering the temperature enough will turn the puddle into solid ice by freezing it.
On occasion, though, there are other ways to cause a phase change. Our word nucleation involves a process that causes a phase change to occur at a much faster rate than usual.
In the case of our exploding pop the first ingredient necessary for nucleation is the pop itself. Soda pop is a liquid with carbon dioxide trapped inside. The thing is, at room temperature and slightly below, carbon dioxide really, really wants to be a gas—but its molecules have been forced under pressure into the liquid. Releasing some of the pressure allows some of the carbon dioxide to change into a gas—which explains the hissing sound as you first open a bottle of pop—but the molecules inside the liquid need a surface of some sort to join up and turn together into gaseous form. This is why you mostly see bubbles along the inside surface of a glass of pop, and also why the pop fizzes in your mouth—in both cases the carbon dioxide molecules are grabbing onto a surface and taking advantage of the situation to change into a gas.
Enter now the Mentos candies. These little things feel really smooth to our fingers, but if you look at them with a microscope you can see that this is an illusion. They are really chock-full of tiny little pits, a little like a miniature version of a golf ball. Each of these pits is an excellent place for desperate carbon dioxide molecules to latch onto, find some friends, and turn into a gas. And this happens FAST.
Now we can start to follow the process that leads to shooting soda pop. As the candies are dropped into the pop, the trapped carbon dioxide molecules very quickly grab onto the pits in the surface of the candy. As they do, they flash into a gas—and as all elementary science students know, a gas takes up more space (or volume) than a liquid does. This means that the carbon dioxide rapidly expands, and in doing so it pushes the liquid on top of it in the only direction available—out of the open hole of the bottle. A pop geyser is born!
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As Mr. Ramsey explained, the whole trick is finding the best way to get the candies into the pop. You want to use around six of them per bottle for the best effect, and you need to get them all in there at once. Just trying to drop them in can be problematic—if you try to do it too fast you’re likely to end up dropping most of them outside the bottle, but if you go too deliberately there’s a good chance the eruption is going to blast right up into your face. We tried several different ways with varying levels of success before Mr. Ramsey brought out…..drum roll, please……the pressurizers.
These little plastic tube-like contraptions actually serve two purposes. First, you can load the candies into them and then screw them onto the open bottle hole. A hinge of some sort (on one it’s a little plastic pin, while with the other we just stick a piece of an index card through a slot) holds the candies in place until you’re ready—then when you release it, they fall into the bottle nice and smoothly. Second, each pressurizer has a smaller hole at the top than the open bottle hole, which makes the pop squirt out with more power and distance, just like holding your thumb over a garden hose makes the water spray farther.
At this point it was time to fire off some eruptions. Each student had brought a pair of unopened bottles of pop, and they each took turns exploding them into the sky with a partner. The students naturally crowded in as close to the pop bottles as possible, so everyone went home rather wet. (Hint: it really makes no difference what flavor or color of pop you use, but whenever possible it’s best to use diet varieties of pop rather than “regular.” Diet pop on skin and clothes and anything else is much less sticky, making for simpler clean up, not to mention an easier car ride home!)
Check out the gallery for photos from our year-end pop geyser celebration. One of our pressurizers was a home-made creation, while the other one is a commercial product that can be found online (search for “geyser tube”) as well as in the gift stores of many science-oriented museums.