What’s really
happening
when we add
Mentos to
Coke?
Now that you’ve tried it yourself, what do you think is happening? Is it a
chemical or physical reaction? Read on to find the explanations about what
you might have observed!
The first thing to clear up is what sort of reaction we’re talking about here. In this case it is a purely physical
reaction, using the very rough surface of the Mentos shell. This rough surface means there are many sites
for bubbles of carbon dioxide (the gas in soft drinks responsible for their fizz) to form. Gas bubbles usually
form on surfaces, such as the side of a glass, as you might have already noticed.
This process is called nucleation, where a small bubble forms at
the surface, and acts as a site for more and more gas to
accumulate until it floats to the surface and pops. The rough
surface of the Mentos means there are plenty of sites for
bubbles to form, many more than on the smooth sides of the
bottle alone. Bubbles of carbon dioxide from rapidly as the
Mentos sinks into the cola, create enough pressure to force
liquid out of the top of the bottle, resulting in the eruption you
see!
How do we know it’s a physical reaction and not a chemical one?
Some people have suspected the acidic nature of cola soft drinks to be behind the reaction, but by
measuring the acidity before and after adding Mentos we can show that no acid was used up by a reaction.
This was done in a report published in the American Journal of Physics in 2008, where a group of university
physics students investigated the different factors which cause this famous reaction. The scientific images
shown below are all taken from that report.
Similarly, a chemical reaction would be sped up by crushing
the Mentos before adding them, but we actually see less of
an effect with crushed Mentos. This is because the bubbles
themselves act as a site for nucleation, as was described
above. The heavier solid Mentos sinks to the bottom of the
bottle much faster, meaning the bubbles which form travel a
longer distance through the liquid, picking up more and more
carbon dioxide as they go.
Chemical reactions go faster when heated however. If you
tried a range of different temperatures for the cola then you
would know this is the case for this reaction too, so how can
we explain this? It turns out that gases are more soluble in
liquids when they are cold, so warm cola would fizz much
more as the gas rapidly comes out of solution, compared to
the cold cola where the gas is happier to stay in solution and
so forms bubbles more slowly.
Why diet cola?
Diet cola uses an artificial sweetener called aspartame, which lowers the surface tension of water. This was
shown in the report by measuring the angle between a water droplet and the surface, using water with and
without aspartame added. This is shown below, where (a.) is just water, and (b.) has aspartame added.
The lower surface tension in droplet b) (with aspartame) means that bubbles can form more easily in that
solution, resulting in more bubbles and a bigger eruption! So while you will still see an effect in any
carbonated drinks, the result won’t be quite so messy.
Why Mentos?
We talked about the rough surface area of Mentos above, which is why it works better than many other
lollies. Below you can see images of the surface of Mentos lollies, which were taken using an electron
microscope. These use electrons to image things rather than light, which makes images at much larger
magnifications possible. Images (a.) and (c.) are of Mint Mentos, while (b.) and (d.) are Fruit Mentos. The
scale bars in (a.) and (b.) represent 0.2 mm, while the scale bars in (c.) and (d.) are just 0.02 mm.
You can see at this scale that the surface is indeed very rough!
The outer shell of Mentos also contains something called gum arabic which, like aspartame, lowers the
surface tension of water, making it easier for bubbles to form. So while other lollies may be as rough (or
rougher in the case of life savers!), the combination of a rough surface and gum arabic make Mentos the
winner for the biggest eruptions. This is also why Mentos works better than rock salt, despite rock salt
having a surface almost as rough as Mentos, and falling to the bottom of the bottle almost as quickly.
References:
Coffey, T S 2008, ‘Diet Coke and Mentos: What is really behind this physical reaction?’ American Journal of
Physics, vol. 76, pp. 551-557.