So far, we’ve talked about how sodas have carbon dioxide in them and that the carbon dioxide doesn’t like to stay in solution. But how do we get it to come out of solution? The key is nucleation!

Nucleation is, in this case, the formation of carbon dioxide bubbles around an impurity (this is often called heterogeneous nucleation). Basically, as I mentioned last week, carbon dioxide wants to come out of solution. The problem is that to effectively come out of solution, it needs a point at which to do this. That’s why you see carbon dioxide bubbles on the side of your glass of soda; the little nicks in the cup, some of which you can’t see, allow a tiny particle of carbon dioxide to come out of solution. More and more particles grab onto the first one, generating a bubble that will eventually break free and float to the surface. Look at it like this:

What to Do:

  1. Pour a cup of soda.
  2. Drop in a pinch of salt. What happens?
  3. Try other things in fresh cups of soda – sand, sugar, marbles, coins, etc. What do you notice?

What’s Happening?

The key to great nucleation a large surface area . Remember that surface area is how much space is exposed is on the outside of a substance.  So, something like a marble is pretty smooth and solid, so there isn’t a whole lot of surface area exposed so not as much nucleation can occur. However, salt is made of very tiny, typically irregular grains, so there is a lot more surface area in salt so there is far more nucleation occurring.

That’s the key to a Mentos soda geyser. The process of Mentos being made includes spraying sugar over the surface to create the hard shell. While the outside of a Mentos may look smooth, this process creates a VERY uneven surface at the microscale (see image below), allowing for a huge amount of nucleation to occur. Test it by dropping a Mentos into a cup of soda and watch the nucleation! Just not inside the Children’s Museum of Houston, please :-)

Scanning Electron Microscope (SEM) image of mint Mentos (left) and fruit Mentos (right) at 200 and 20 micrometers (about 100 and 1000 times smaller than a penny, respectively).

(Image: T Coffey/Dewel Microscopy Facility/AAPT)