Scientists love explosions. They love using explosions, like the popular Mentos and Coke reaction, to get people excited about science. Add Mentos candies to a bottle of Diet Coke, and it will erupt in a fountain of bubbles, spraying 5-10ft high.
After the fountain subsides, students brainstorm: how and why did this reaction happen? The simple answer is that the rough surface of the candies provides lots of little growth sites for bubbles. The truth, like many things in science, is a bit more nuanced. In June, 2008, a physics professor from Appalachian State College and her students published a paper demonstrating all the factors that effect the reaction. Tonya Coffey’s class tested a variety of hypotheses by repeating the reaction with different kinds of soda, different candies, and other substances that have similar characteristics to the Mentos, like the rough surface of rock salt or sand.
They tested the rough surface hypothesis by comparing Wintergreen Lifesavers to Mentos. Both candies, the rock salt, and even dish soap, caused bubbling eruptions. Anything dropped into the soda can break the fragile bonds between water molecules, allowing carbon dioxide bubbles to grow. But not all of the eruptions were the same size. Under a microscope, the surface of the Lifesavers appears 5 times as rough as either the mint or fruit Mentos, which are very similar. However, the eruption with the Lifesaver is half the size of the average mint or fruit Mentos eruption.
The difference they discovered is that Mentos are very dense, and sink to the bottom of the soda bottle 33% faster than the Lifesavers. The bubbles created at the bottom now have to travel all the way back to the top of the bottle to escape. The bubbles create a chain reaction, each one creating more as they disrupt the cohesion of the water molecules. The bubbles that travel from the bottom create more pressure, causing a more powerful the eruption. Coffey compares the Lifesaver’s reaction to a volcano slowly oozing lots of lava, instead of spraying high into the sky like the Mentos.
The second factor Coffey’s team found that plays a role in the high spray of the Diet Coke and Mentos or Fruit Mentos is the fact that the Coke is diet. The artificial sweetener, aspartame, weakens the bonds of the water molecules in the soda. This makes it easier and faster to for the Mentos to break those bonds and start building bubbles. A bottle of regular Coke erupts too, just not quite as high.