This is a very easy experiment and one that all kids love. It’s messy so best to do it outside. There are also lots of variations you can test out:
- Diet versus normal soft drink
- Coke versus other soft drinks
- Mentos versus other sweets
Hint: for the biggest explosion use diet soft drink and fruit Mentos.
The Science Behind It
The Coke and Mentos experiment is a popular one that has loads of youtube videos on it. The following is a great one explaining what’s happening:
Water molecules like to be next to other water molecules, so basically anything that you drop into the soda that disrupts the network of water molecules can act as a growth site for bubbles. And if you have rough candy with a high ratio of surface area to volume, then there’s more places for the bubbles to go. – Tonya Coffey
Diet works best because it contains Aspartame
Greatest reaction was with Diet Coke and Coke Zero because they both contain the artificial sweetener – Aspartame. This is because Aspartame lowers the surface tension of the drink, making it easier for the bubbles to form. This allows the reaction to take place more quickly. You will notice in the video above, the experimenter hardly has any time to drop the Mentos sweets before the liquid erupts.
Chemical or Physical?
Contrary to some explanations, this is not a chemical reaction but a physical one. Lots of speculation on the internet suggest that it is an acid-base reaction, again this is incorrect. A test of pH at the end of the experiment reveals that the soft drink’s pH is not altered by the reaction. Additionally, there are no ingredients that provide the “base” in an acid-base reaction.
Why does Mentos work better than other sweets?
Soft drinks are fizzy because carbon dioxide has been dissolved into the liquid under pressure. When you open a can or bottle of soft drink, the carbon dioxide in the drink slowly rises to the surface and escapes. By adding Mentos, we make this reaction occur more quickly because the rough texture of the Mentos provides nucleation sites for the carbon dioxide bubbles to form. Mentos works better than other sweets because it’s heavy and falls to the bottom more quickly. Additionally, Mentos sweets contain a sugar coating that further reduces the surface tension of the drink, facilitating bubble formation.