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