It’s one of the most popular science demonstrations: Pop a handful of Mentos candies into a bottle of Coke, and a fountain of bubbles rapidly spurts from the open bottle, often over a metre into the air. Originally the explanation for the effect was thought to be quite simple. However, scientists are finding that there’s more to this spectacular demonstration than meets the eye.
The basic science of the Coke and Mentos reaction is fairly straightforward. In fact, it’s not really a reaction at all – or at least it’s a physical reaction rather than a chemical one. Carbonated drinks get their fizz from dissolved carbon dioxide, which is pumped into the bottles at high pressure to make it dissolve. Your average two litre bottle of Coke will contain about fifteen grams of dissolved carbon dioxide.
When you open a bottle of a carbonated drink, you release the pressure. As the carbon dioxide is less soluble at lower pressures, it starts to leave the drink in the form of small bubbles. When you add Mentos to the bottle, you’re drastically speeding up this process. Though the surface of a Mentos candy may look smooth, at a microscopic level it’s full of pits, peaks, and craters, like a miniature version of the surface of the moon. These pits, peaks, and craters are referred to as nucleation sites. They provide a surface for the carbon dioxide bubbles to form on, and allow them to form much more rapidly.
That paltry fifteen grams of carbon dioxide in the bottle might not sound like much, but as it comes out of solution in the bottle it can expand to take up up to 8 litres in volume. This explains why the drastically increased formation of bubbles after adding Mentos to the bottle leads to the geyser of Coke shooting out of the top moments later – there’s nowhere else for the bubbly liquid to go but up!
Science teachers are well aware that some types of carbonated drinks work better than others for this demonstration; for example, Diet Coke usually gives a higher fountain than regular Coke. Previously, scientists have tried to investigate why this is, and they suggest that the artificial sweeteners in diet forms of carbonated drinks, specifically aspartame and benzoate, might be responsible. Their suggestion was that these compounds lower the surface tension of the liquid, which allows bubbles to form quicker. Sugar-sweetened drinks tend to be more viscous, which likely slows bubble formation, leading to smaller fountains than for their diet counterparts.
More recently, chemists at Spring Arbor University in the United States tested a range of different carbonated drinks and found that the height of fountains varied between different products. Generally, they found that carbonated water gave the smallest geysers, with sugar-sweetened beverages giving better results, and diet beverages better still. They suspected other compounds dissolved in the drinks might be affecting fountain height, so they carried out further tests on these compounds in isolation.
By dissolving quantities of the compounds in carbonated water, they were able to test each compound individually. Five common compounds in carbonated beverages were tested: aspartame, benzoate, linalool, citral, and citric acid. Their results showed that increasing the levels of all of these compounds increased the heights of the fountains obtained by as much as six times when Mentos were added, and at quantities usually found dissolved in commercial carbonated drinks. The exception was citric acid, which did increase fountain height too, but which required slightly more than usually used in drinks.
The results aren’t fully explained by the previous theory relating to surface tension. This is because some of the compounds tested actually increase surface tension when they are dissolved in water. The proposed explanation for this is that the dissolved compounds may actually be affecting how the bubbles themselves behave, and it is this combined with factors relating to surface tension that influences fountain height.
Investigating further, it was found that when the substances were dissolved in water, the bubbles produced were smaller. Bubble size, it turns out, is inversely proportional to the height of the Coke geyser; the smaller the bubbles, the greater the height achieved. The dissolved substances in the drink were stopping the smaller bubbles combining into bigger bubbles. When the bubbles are smaller, there is a greater surface area for the carbon dioxide still dissolved in the solution to escape into, so the degassing of the carbonated drink happens faster – giving a higher fountain.
Of course, the different substances dissolved in carbonated drinks don’t act in isolation, and they may also interact with one another to increase or decrease the height of the coke fountain. This may be the focus for further experiments. For now, it’s clear that there’s still plenty to learn about the best way to make the biggest Coke and Mentos fountain!
This graphic is based on the following paper: New demonstrations and new insights on the mechanism of the Candy-Cola soda geyser – T S Kuntzleman, L S Davenport, V I Cothran, J T Kuntzleman, D J Campbell; J Chem Educ; Link: http://dx.doi.org/10.1021/acs.jchemed.6b00862
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