Before we go into the stuff you will need, lets take a quick look at what the fuss is all about. In this video, these two guys reproduce the Las Vegas Fountains show, using diet coke & mentos.

Great video, isn’t it?

So are you ready to try this at home? As with any experiments, think about child safety!

Here is what you will need:

1. An outdoor space that you don’t mind getting covered in diet coke.
2. A packet of Mentos. Mint mentos are often used, but scientists in the US have also used fruit ones. Which ones produce the biggest geyser of coke? Why not find out for yourself – see the section at the end of the page entitled “Doing more coke & mentos science with your kids”.
3. A 2-liter (if possible) plastic bottle of diet coke. If you can find bigger bottles, you will probably get a bigger reaction! Keep the bottle of coke warm, rather than in the fridge. You should get a better reaction with warm coke.
4. A piece of A4 paper and a small piece of card.

Diet Coke & Mentos Experiment Instructions

1. Create a paper tube, down which the mentos can pass easily. The paper tube should be just big enough to go over the opening of the coke bottle. At the same time, cut a small square piece of card that you can place over the opening of the coke bottle, and easily remove it. About 5 cm square is a good size.
2. Go outside to an area where you don’t mind the coke getting everywhere. I have seen this experiment done indoors, and its not something I recommend.
3. Take the lid / cap off the bottle of coke.
4. Place the card over the opening hole in the coke bottle and position the paper tube over the card.
5. Drop the mentos into the paper tube. The card should prevent them from falling into the coke.
6. Remove the card so that the mentos fall into the coke, and get well back…. QUICKLY.

The reaction should be near instantaneous. The fountain of coke will shoot out of the bottle, and it can go very high.

The Science Behind the Diet Coke & Mentos Experiment

While the science behind this experiment has been debated since the late 1990s, scientists in the US have come up with the reasons why mentos plus coke = coke geyser.

One of the ideas that was suggested previously was some kind of reaction between the gum arabic and gelatine in the sweets with caffeine, potassium bonzoate and aspartame in the coke. However the US scientists tested this, and found that these factors were not important to the reaction.

What the US researchers did find were the following main factors that are involved:

1. The rough surface on the candy
2. The weight of the mentos which move very swiftly to the bottom of the bottle.
3. The dissolved carbon dioxide in the diet soda.

The size of the mentos geyser created is dependent on the rate at which Carbon Dioxide bubbles are formed in the coke.

The rough, dimpled surface of the mentos encourages bubble growth. You can see this by dropping a peanut in coke – it sinks, slowly becomes coated in bubbles as the CO2 bubbles form, and the peanut then rises to the top because of the boyancy of the air in the bubbles.

The scientific reasons why the pitted surface causes the bubbles to form is a little, well, scientific. Its not something that kids will understand easily, but if you want to know the answer, here it is:

The rough surface of the mentos has a huge surface area to volume ratio with all these tiny “pits”, and this “surface” interferes with the water molecules (which like to be next to each other) causing bubbles to form at the dimple sites. The more dimples, the more bubbles. Therefore the more pitted the surface, the more disruption that occurs and the better the geyser.

Gum arabic in the mentos also has an affect – lowering the surface tension in the liquid, which again encourages bubbles to form around the mentos. The scientists in the US found that sweets without the gum arabic coating did not produce the substanital geysers seen with the Mentos sweets.

Another contrinuting factor in the mentos geyser is the weight of the mentos. Since they are quite dense, they sink quickly and as they do, they start creating bubbles, which rise and cause more bubbles to be formed, thereby accelating the bubble formation in the reaction.

Here is a video explaining the main ideas:

Doing more coke & mentos experiments with your kids

If you had fun with this experiment, there are several things you could try with your kids to see what effect they have on the reaction – a kind of science project that could go on for days! Your kids can try to estimate the height of the geysers under different conditions (estimating anything is a great skill for kids to learn as well) outlined below and find the record breaking combination for the best coke & mentos geyser.

Here are some ideas on modifications you could try:

• Try warm / cold diet soda and see which one produces the highest fountain. As with any reaction, heat tends to speed things up because it increases the collisions of particles in the reacting substances. You should therefore see a better reaction with warm pop than cold. A word of caution. Be careful how you heat up (or should I say warm up) your pop. As the pop warms, it will expand and cause more pressure in the bottle. I would recommend putting the pop in a bucket of hot water for 10 minutes, but please do be careful when it is time to do the experiment – take the bottle lid off carefully.
• Since the reaction is down to the surface pitting of the mints, try another experiment with Fruit Mentos instead of Mint Mentos. The fruit mentos have a smooter surface, and therefore should not create the volume of bubbles that the mint mentos do. This will lead to a smaller reaction.
• Try adding other things to the soda. e.g. rock salt is suppose to work well. Just use course sea salt if you have it.
• Try caffeine free coke, and normal regular coke. Is there a difference in geyser height? Scientists in the US said that the caffeine-free coke worked just as well.
• Since the number of pits in the mentos is important, it should follow that the geysers will increase in size as you add more mentos to the bottle. Try adding 5 in one experiment and 10 in the next. Did the geyser increase?
• The weight of the mentos is said to be a contributing factor in the reaction. “Lighter mentos” should not cause as much of a geyser as heavier ones. Can you think of ways to increase or decrease the rate at which the mentos fall in the coke? If you need a clue, think of a skydiver before and after pulling her parachute. Maybe a flattened mento will fall slower?
• Why not try other carbonated drinks to see what size eruption you get?
• Think about safety with this suggestion, but one of the alternatives to the coke geyser type of experiment is actually to use the pressure of the exiting liquid to fuel a rocket launch from the ground. Can you design an experiment to create a rocket like this? Maybe I’ll show you how in another science experiment