The World-Famous Mentos Geyser

In this week's book, Alvin Ho isn't afraid of explosions and loves things that go up in the air. So I thought we'd do a classic science trick that Alvin would probably love--The World-Famous Mentos Geyser! Okay, it's not technically an explosion (or a geyser), but it's the closest thing I can safely encourage kids to try. :)

Ingredients:

• 2 Liter bottle of soda (diet will make the mess a little less sticky, and it works a little better if it's left at room temperature instead of cold)
• 1 roll of Mentos candy
• paper, index card (we used a used-up gift card), and tape
• large, flat outdoor surface
• microscope (optional)

Procedure:

1. Unwrap your candy. Roll and tape your paper so it's just the right size to slide the Mentos inside (but not to tight that they won't slide out easily when you're ready.) Use an index card to keep the Mentos from falling out the bottom.
2. Open the lid of the soda and place it out in the open.
3. Put the tube of candy over the mouth of the bottle, then quickly remove the card so the candies fall into the bottle.
4. Stand back and watch the geyser!

(Note: My assistant does have shorts on under her dress. But no, we're not very ladylike. :)

How it works:

The bubbles in your soda are carbon dioxide, and they want to escape. But the carbon dioxide molecules have to find each other and form bubbles before they can float to the surface. To do this, they need a nucleation site. That's a big term and you'll sound smart when you say it, but it's basically just a spot where a bubble can form. It's pretty hard for bubbles to form in a smooth plastic bottle or the side of a glass. (Which is good--otherwise your soda would lose its fizz!)

The great thing about Mentos is that they're not as smooth as they look. If you study them under a microscope, you'll see that they actually have tons of tiny craters, almost like the moon. And every one of these craters can be a nucleation site where bubbles form.

When the candy drops in all at once, the bubbles begin to form in all those tiny craters. And when they all try to escape from the bottom of the bottle, it's a race to get out and a geyser forms!

For more information, check out this MythBusters video and this chemistry site. And if you're feeling really smart, check out this Mentos and Diet Coke article from the American Journal of Physics. (Seriously!)

Good luck! If you try this one, I'd love to hear about it. What soda works best? What happens if you add a little dish soap to the soda right before? I have a theory about this one... :)

And just for your viewing pleasure, here's one more successful geyser and a couple of outtakes: