Mentos and Diet Coke

Mentos and diet coke has become a well known science experiment as it is both interesting and fun. When mentos is dropped into a bottle of coke the resulting reaction between the coke and the mentos causes a jet of foaming coke to erupt out of the bottle.

The main cause of the eruption of mentos and diet coke is not due to a chemical reaction but a physical reaction.  Soda contains dissolved carbon dioxide; this is what gives the soda its fizz. When carbon dioxide is dissolved in water it forms carbonic acid and this acid will decompose in the soda separating into water and carbon dioxide and this is when the soda becomes flat. All this dissolved CO2 gas can over course be released within a short period of time when something is there to catalyse the process, e.g. a mentos.  

Mentos are covered in small indentations because of the way the outside sugar layer was applied when the sweet was produced. These small indentations act as nucleation sites letting bubbles form there and then escape into the liquid.  The mentos sinks to the bottom of the soda bottle due to its high density compared to the surrounding liquid. This causes the carbon dioxide to be released resulting in a build up of pressure thus forcing the liquid above to shoot out of the bottle.

The reason I recommend using diet coke instead of another carbonated beverage is because the chemicals in diet coke help the release of carbon dioxide to be more rapid. This causes the liquid to jet higher into the air compared to other sodas.

One of the chemicals in coca cola’s diet coke recipe facilitates the formation of bubbles by lowering the surface tension of the water. This means it will take less energy for the bubbles to form and so they will form rapidly. Also use white mentos in the reaction not green ones since the green ones are glazed and smooth so they do not have any nucleation sites on their surface. 

Mentos and coke reaction
Mentos and coke reaction

Mentos and coke reaction Mentos and coke reaction Mentos dropper. About to pull the pin

Experiment

You will need:

  • A two litre bottle of diet coke 
  • 6 white mentos (or more)
  • A tube
  • (safety goggles)

Pick an open area to let off the mentos and diet coke geyser. It might also be wise to do the experiment on grass so you don’t have to worry about cleaning up all of the spilt soda. Now place the coke bottle on the ground and unscrew the bottle cap. Place the 6 mentos the tube (use more or less if you want) and hold the one opening closed so they do not fall out. Hold the tube above the opening of the bottle and then release the mentos and step back. 

A remote mentos dropper may assist you to quickly drop the required amount of mentos quickly and from a distance. If you don’t have a tube that fits the requirements you can make one out of paper by shaping it into one. 

To make the remote mentos dropper you will need:

  • 1 A4 sheet of paper
  • Sellotape 
  • A hair pin (or a paper clip, match stick, etc.)
  • A piece of string (as long as it needs to be)

Cut the A4 sheet of paper in half length ways. Then I used one of the un-opened mentos tube packs and made a tight rolled around it with the paper. At that point I adjusted the diameter of the tube so that it would snugly sit inside the opening of the bottle. Once I had made sure it fitted I sellotaped the paper securely so that it formed a sturdy tube. Now "drill" two holes into the bottom of one end of the tube (2 or so centimeters, 1 inch from the end) so that they are opposite each other. These two holes will be were the hair pin will be inserted into and will act as a barrier preventing the mentos from dropping into the coke. Once you are ready you can pull the string which is attached to the hair pin and the mentos will fall. This will insure you are well away from the foaming mentos and diet coke geyser.

Ideas for a science fair project

Does the geyser go higher when the coke is warm or cold?

When soda and mentos are combined carbon dioxide is released so rapidly from the beverage that a stream of soda will shoot up into the air. Try this with coke at different temperatures (e.g. 10⁰C, 20⁰C, 30⁰C, 40⁰C, 50⁰C...). Then draw a scale on the side of the wall going up in increments. You may want to use a camera with a fast shutter speed to help you to discern how high the geyser reached.  If you have a good set of eyes just watch carefully and then record the height of the geyser.

Does mentos and diet coke give the best reaction or are there better sodas out there. You could test different types of soda to see which one reacts the best with the mentos resulting in the geyser that shoots the highest.


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