Part 1: Mentos and Diet Coke Eruption
Materials
- One pack of mint Mentos
- One two-liter bottle of Diet Coke
- Paper tube made by rolling a half-sheet of paper loosely around the Mentos roll and securing with tape – or you can just use the Mentos label
- Index card or other piece of thin cardboard
- Measuring tape
- Towels, water (to clean up)
Before You Begin
This is going to be messy. I strongly recommend that you perform this experiment outside. If you can’t go outside, you can do it in the bathtub or shower using only one or two Mentos. You can also use soda water in place of Diet Coke. The fountain won’t be as high, but it will be easier to clean up! Make sure you have plenty of water on hand to rinse down the area after your experiment.
Method
- Slowly open the 2 liter bottle of Diet Coke.
- Load the paper tube with four Mentos.
- Place the index card over one end of the tube and invert it.
- Set the card and the tube with the Mentos inside on top of the open mouth of the bottle, making sure the opening of the tube lines up with the mouth of the bottle.
- Quickly remove the index card, allowing the Mentos to drop into the bottle
- Step back quickly!
- Record the height of the geyser
- Optional: repeat using a new bottle of Diet Coke and more Mentos – how high can you go?
- Enter your data on the spreadsheet provided. Once we have data for the entire class, we will do some statistical analysis on our results.
Pointers
- Determining the height can be a little tricky in the moment. It helps to have two people – one to load the bottle, and one to observe. Or set up a video camera and record your experiment so you can estimate the height after the fact.
- If you can, the best way to determine fountain height is to make markers at one-foot intervals on a wall, tree, or pole (see green tape markers on garage door in image below). You can use masking tape or chalk to make the marks. This is easier than trying to hold up a measuring tape.
- Or, you can use a person of known height for a reference. Was the geyser waist-high? Chest-high? Above the head?
- It’s best to put the bottle on a flat surface if possible so it doesn’t tip over.
- Make sure the Mentos can fall freely from the tube before you begin. If the tube is wet or crushed, the Mentos can get stuck inside. If this happens, make a new tube.
- Try not to shake the bottle before you open it, or you will lose some CO2 before the experiment begins. Also, it’s difficult to get the Mentos to go down into a bottle that’s already foaming over.
Just for Fun
If you have someone to help you (or a trusty tripod), record a video of your eruption and turn it in! I’ll make a compilation of “greatest hits” for the website.
Video: Diet Coke & Mentos | MythBusters (1:57)
NARRATOR: The Mythbusters are going to have a blast.
ADAM: The most common theory about what's going on in this reaction between the candy and the soda is what's called nucleation. Basically the idea is that the surface of the candy is covered with microscopic pits and lots of more surface area than you can actually see and each little pit, each little corner provides what's called a nucleation site or a place where carbon dioxide bubble can form and escape.
NARRATOR: Look at a Mentos close up and it's like the surface of the Moon and that might be the candy key. Drop one in cola and every tiny crater provides a site where a CO2 molecule can change to gas. Because Mentos are so pitted the theory is that millions of CO2 bubbles are formed in a very short space of time, and because the candy sinks and this rapid release of gas happens at the bottom of the cola you get that famous fountain.
To test this nucleation theory they're going to start with a control. One regular mint Mentos dropped in soda water and sure enough it gets the bubbly party started. Now to do a comparison.
JAMIE: These two candies are made by the same manufacturer and as far as I'm aware even using the same process, but the colored version of this actually has a glazing over it. It's a wax coating or a sealer that inhibits the nucleation process that the other one achieves quite readily.
NARRATOR: The shiniest surface should lower the nucleation sites meaning less of an immediate eruption.
JAMIE: That's not doing anything more than the wall of the plastic bottle itself.
NARRATOR: Sure enough with the smooth mentos there ain't no Wiz with the fizz. The Mythbusters can say they finally cracked the case of the candy and cola cascade.
Video: Science of Mentos - Diet Coke explosions explained (1:26)
The fountain of foam produced when you drop mentos into diet coke has been seen millions of times on YouTube. Until now there's not been a scientific explanation. A study in the US has identified the two main factors involved. They turn out to be one the roughness of the surface of the sweet and two how fast it plummets to the base of the bottle. Students compared the effects of fruit and mint Mentos as well as various other ingredients. Here they are testing sound. previous theories put the reaction down to the acid in the coke or the gum arabic and gelatine in the sweets these explanations have not been properly tested. With no ph change occurring and acidic reaction was ruled out as a cause as was gum arabic following less impressive results when tested independently as can be seen here. Instead the fountain effect was explained by the rough dimply surfaces of the mentos encouraging carbon dioxide bubble growth. These are scanning electron microscope images of the mint Mentos on the left and fruit Mentos on the right. The scale bars represent lengths from 20 to 200 micrometers. Mentos are also fairly dense and sink rapidly quickly creating bubbles seed further bubbles as they rise.
[Music]
Video: Original Mentos Diet Coke Geyser - Cool Science Experiment (2:13)
MAN: This is really good. All right. So here's the setup. Mentos, and the reason we choose Mentos is because they have this chalky like quality to it.
WOMAN: Right.
MAN: What we want is.. we're just gonna put them into the test tube so they go down to the test tube like this. So there's your test tube got it. 13 Mentos is just in case you're counting. All right, so there it is. So now let me open this up here. And yours, you can open yours up as well all right okay. So and we're just using diet just because it's less sticky when it finally there's a disaster.
WOMAN: Oh when there is...
All right, so now here's what's going to happen. We put the card on. We turn the whole thing upside down we're gonna put it over the top. We're gonna position it so it's right over the top. You got it? Right there, okay. All right, now when we pull it out those are going to fall real fast and when they do all of the bubbles that are inside are going to rush to them and we might see a little eruption. Are you ready? Alright so we're going to go three, two, one pull ready three, two, one pull good ...ahh!
Tell me that's not awesome. So sorry look at you you! Wasn't that fun? That was fine that's the best I have ever seen! Is that are.. you good? Alright so. So just ask them is it better for diet or regular go ahead and ask?
WOMAN: Is it better for diet or regular?
MAN: Let's go find out. Come on let's go works in a big way it does not let you you go there I'm going go here 'cause you that's this is, this is the regular. All right.
Okay we're good. Alright so you remember then the carbon dioxide is coming out. Alright hey so this is ... you're okay alright so turn it over and remember they're gonna come rush into it. So let's do it together. Ready three, two, one, pull...
Awesome! Mine didn't even fall in ... get out of the way I'll do this with me so here we go. Three, two, one .. now that is awesome! Is that great? So here, yeah you look great no you look awesome, and guess what I put it all on the website so a teacher can be amazing to learn about states of matter and gases. I'm so sorry, look at you, you're a mess.
The Science
A bottle of soda contains dissolved carbon dioxide (CO2) under pressure. When you remove the lid, the pressure is released, and the CO2 exsolves in the form of tiny bubbles. When exsolution occurs faster than the gas can escape, the soda gets whipped up into a foam that quickly overflows the confined volume of the bottle – if you have ever shaken or dropped a bottle of soda before opening it, you have probably observed this effect yourself. In this experiment, the Mentos encourage the rapid formation of bubbles by providing a nucleation site. In the absence of a nucleation site, the CO2 gas must overcome the surface tension of the liquid before it can form a bubble, which inhibits the process a bit, especially at the beginning. Mint-flavored Mentos have a pitted surface with lots of surface area, which provides plenty of nucleation sites for bubble growth. The more Mentos, the more nucleation – hence, a soda eruption! It is less clear why Diet Coke works better than regular Coke, but based on observation this seems to be the case. Some people have suggested a chemical reaction involving the artificial sweeteners. However, any carbonated beverage will produce a fountain when Mentos are added, some will just be more dramatic than others. Incidentally, fruit-flavored Mentos do not produce an eruption. This is because they have a smooth waxy coating that does not provide nucleation sites for bubble formation.
Lab Worksheet
Download the Excel Spreadsheet [2] to enter your experimental results
Download and complete the Worksheet for Lab 2: Degassing [3]
You will need to submit the results spreadsheet and the complete worksheet to the Module 2 Lab Assignment in Canvas.