The Coke-Mentos experiment is one of the most popular science experiments – and also one of the most well-known. It’s relatively easy to do because the materials are readily available: with nothing but a few Mentos and a bottle of Coke, you can trigger an immediate, explosive reaction, making this a very simple yet very fun (if slightly messy) experiment.
In this post:
What Happens When You Mix Coke With Mentos?
When Mentos react with Coca-Cola, the following reaction is explosive, rapidly forming a fountain of pressurised Coke that then spurts out of the bottleneck and shoots into the air. This spectacle is mainly caused by a physical reaction that rapidly releases the dissolved carbon dioxide from the beverage.
Carbon dioxide is the force that pushes the liquid out of the bottle in a fraction of a second. This happens because the surface of Mentos has thousands of microscopic pores, peaks, pits and craters in it – so what we see to be a smooth outer shell is pure deception.
These microscopic irregularities on the Mentos’ surface serve as nucleation sites where carbon dioxide bubbles form. Thousands of microscopic carbon dioxide bubbles accumulate in these pockets, which then leaves gaps inside the bottle. This forces the liquid to overflow and shoot out.
Speed and pressure are also important factors in the explosivity of Coke fountains. The formation of carbon dioxide bubbles is like a chain reaction that exponentially accumulates, causing pressure to build in a fraction of a second. The narrow neck of the bottle also intensifies the pressure and speed because it forces the flow rate to increase. For all the liquid to leave through the opening, the upstream pressure then also has to increase, redirecting the flow of the Coke upwards and – you know the rest: explosion.
There are many other factors that can affect the height of a soda fountain when Mentos react with Coca-Cola, such as the viscosity of the liquid and presence of other chemicals in the mixture. For instance, if citric acid is added to a soda mixture, the fountain height will actually increase to up to six times its normal height.
What Chemical In Mentos Makes Coca-Cola Explode?
It’s not only the surfaces of the Mentos that react with Coca-Cola. Aside from the microscopic structures on the outside of the Mentos shells, there are several chemicals that make Coca-Cola explode in this way. These chemicals are found in the shell itself as well as in the soda mixture, and they contribute to the formation of pressurised foam. Here are some of the chemicals that make up the shell:
- Sugar: The Mentos shell has sucrose and glucose components, which are solid within a normal temperature range
- Aspartame: This is an artificial non-saccharide sweetener that is 200 times sweeter than table sugar or sucrose (C12H22O11). While it’s chemically and molecularly similar to sugar, it has a nitrogen in its composition: C14H18N2O5
- Potassium benzoate: This is a food preservative that prevents the growth of fungi and bacteria. It’s also the potassium salt of benzoic acid
These ingredients act as surfactants and help accelerate the release of carbon dioxide gas. Surfactants lower the surface tension between two liquids, a liquid and a solid, or between a gas and a liquid.
So, when a Mento is dropped into some Coke, the acidity of the soda mixture quickly dissolves its shell, releasing the chemicals. Their surfactant properties then lower the surface tension of the Coke, which breaks apart the water molecules and, in doing so, allows carbon dioxide bubbles to form more readily. With this in mind, the chemicals in Mentos also serve as foaming agents by facilitating the rapid release of CO2 gas.
Mentos don’t only react with Coca-Cola. Various types of carbonated beverages react differently with Mentos: Seltzer water has the weakest reaction while, very specifically, Diet Cherry Dr. Pepper has the strongest reaction. This was demonstrated in an experiment that tested 15 types of carbonated beverages with Mentos. Each bottle tested had a two-litre capacity, and eleven Mentos were introduced to each bottle. A two-litre bottle has about 15 grams of dissolved carbon dioxide, which, under the right conditions, becomes 8 litres of carbon dioxide gas in just a few seconds.
The results of the experiment are shown in the illustration below:
The types of solutes in the soda mixture also have effects on the height of the fountain. An experiment on Seltzer water, adding various types of solutes, produced the following results:
- Aspartame produced a fountain up to three times the original height
- Benzoate produced similar results to aspartame
- Citral produced a fountain up to four times the original height
- Linalool produced similar results to citral
- Citric acid produced a fountain up to six times the original height
What Is The Chemical Structure Of Coca-Cola?
Coca-Cola isn’t a single chemical compound but rather a mixture of different chemicals. This mixture was first formulated by an American pharmacist, John S. Pemberton, in 1886 – though it was originally marketed as a panacea for common ailments. The original ingredients of Coca-Cola contained cocaine from coca leaves and caffeine-rich extracts from kola nuts.
The present-day ingredients of Coca-Cola classic are a little different:
- Carbonated water: You may not think it, but about 90% of Coke is water. Dissolved carbon dioxide accounts for just 0.75% of the total mass.
- Sugar or sweeteners: Sucrose is the ingredient that provides the sweet taste in Coke classic. Coke Zero and Diet Coke don’t contain sugar, however. Instead, they contain artificial sweeteners, like aspartame.
- Food colouring: The characteristic caramel colour of Coca-Cola is just artificial colouring. Without this food colouring, the beverage would just look like water – this is the secret behind Coca-Cola Clear.
- Phosphoric acid: The pungent taste and tartness of this well-loved fizzy drink actually comes from the phosphoric acid. But this chemical isn’t only used because of its tangy flavour; it also helps prevent the growth of bacteria and mould.
- Caffeine: This is what causes the slightly bitter taste of Coke. It’s not as strong as the caffeine in coffee, however, which contains about three to four times as much.
- Natural flavours: These are the cryptic essence of Coke. The exact ingredients here are part of the protected proprietary secret formula of the company, which is very difficult to duplicate.
Why Do Diet And Coke Zero Have A Better Reaction With Mentos?
Diet Coke and Coke Zero react better with Mentos than regular Coke does because of the absence of sugar. A lack of sugar makes the soda mixture less viscous, given the presence of sweeteners, like aspartame, lowering the surface tension even more than usual. This in turns means that the carbon dioxide gas is more rapidly released. The nucleation process is also faster, leading to higher gas pressure.
We can see how much better Diet Coke and Coke Zero react with Mentos just by looking at the experiment we showed earlier: both of these sugar-free sodas produced fountains that exceeded 2.5 metres, while the fountain produced by regular Coke barely reached 1.5 metres. So, in a nutshell (or, more aptly, a Mentos shell), soda mixtures with more sugar in them are more viscous, making their reactions less powerful.
The Coke-Mentos science experiment is certainly fun to do – but it’s also messy, so we recommend doing it outside or in an easily-cleaned space, like a bath! If you’re still on the fence about trying it, it may help to know that the experiment also provides some basic insights on the chemical and physical properties of reactants – so it’s an educational experiment, too!