There has been a lot of discussion online about why Diet Coke and Mentos make such an interesting combination. What everyone can agree on is that the carbon dioxide that has been compressed into the soda escapes so rapidly that the pressure pushes the soda out of the bottle. It’s like shaking a bottle before you open it, but even more dramatic.
Gum Arabic? Not So Fast.
Several people theorized that a substance called gum arabic in the Mentos breaks the surface tension of the soda, allowing the carbon dioxide bubbles to escape rapidly. This explanation doesn’t completely work. One thing that casts doubt on the gum arabic theory is that several items (like pennies!) that contain no gum arabic whatsoever also cause soda to foam violently. Several chemists have since stepped forward to say that while gum arabic may contribute to the reaction, that can’t be the only cause.
These chemists are saying that the primary cause is physical, not chemical. Their explanation: nucleation sites. If you have a liquid that is supersaturated with gas (like soda, which is pumped full of carbon dioxide), a nucleation site is a place where the gas is able to form bubbles. Nucleation sites can be scratches on a surface or specks of dust – anywhere that you have a high surface area in a very small volume. That's where bubbles can form.
