Demonstrations to school students can easily degenerate into the following format. First, one does something spectacular such as the Coke-Mentos fountain or the barrel crush. Second, one tells the student how it works. I confess I have often done this. However, this is actually terrible because it reinforces the misconception that science is a noun not a verb. It teaches nothing about the scientific method.
This week my wife and I demonstrated the Coke-Mentos fountain to a group of kids at a holiday club that my church was running. In order to promote critical thinking we did some comparative measurements. The fountain was done for diet Coke, Solo (a lemon drink), and generic brand (Coles) Cola. We also compared Mentos bought in the USA (on my recent trip) and in Australia. It turned out that the former is much more effective. I later learnt that we had been scooped in this important scientific discovery. It had already been published on YouTube!
I also discovered there is some nice literature on the subject.
Mentos and the Scientific Method: A Sweet Combination in the Journal of Chemical Education.
Diet Coke and Mentos: What is really behind this physical reaction? in the American Journal of Physics. They have some impressive apparatus for making quantitative measurements. They also found that playground sand was almost as good as Mentos.
They report surface analysis studies of the Mentos, highlighting the importance of the surface roughness for nucleation sites for CO2 bubbles.
The Ultrasonic Soda Fountain: A Dramatic Demonstration of Gas Solubility in Aqueous Solutions in the Journal of Chemical Education
“Can we do That Again?” Engaging Learners and Developing Beyond the “Wow” Factor in the Science Education Review.
Finally, having good apparatus helps. From Steve Spangler science we purchased a Geyser Tube which feeds the Mentos into the Coke. With the recommended 7 US Mentos we observed fountains of 2-3 metres!