In Part 1 of this lab activity, you will conduct the famous "Mentos and Diet Coke eruption", under more-or-less controlled experimental conditions. In Part 2, you will make some degassing calculations of your own. First, you will calculate the amount of CO2 released from a 2-liter bottle of Diet Coke. Then you will apply the same principles to calculate the amount of SO2 released during the 1783-84 eruption of Lakagígar.
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.
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.
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.
Download the Excel Spreadsheet [3] to enter your experimental results
Download and complete the Worksheet for Lab 2: Degassing [4]
You will need to submit the results spreadsheet and the complete worksheet to the Module 2 Lab Assignment in Canvas.
The idea here is to determine the mass of CO2 you released into the atmosphere during the first part of your experiment. Watch your unit conversions!
First, a few assumptions:
We start by determining the total mass of CO2 present at the beginning of the experiment (prior to opening the bottle). In order to do this, first you will need to determine the mass of Diet Coke. Use the graph below to determine the density of water at 20˚C; we will assume your Diet Coke has the same density. Note that 1 cm3 = 1 mL.
Solubility is the amount of a compound that will remain in solution under a given set of conditions. Use the graph below to estimate the solubility of CO2 in water at 20˚C and atmospheric pressure.
The amount of CO2 released is given by the total amount present prior to opening the bottle minus the amount retained after the degassing experiment.
We can use the same approach to calculate the mass of SO2 released from the lava during the Lakagígar eruption. First, we need to estimate the mass of SO2 dissolved in the magma prior to eruption. But how does one determine the concentration of a volatile component prior to degassing, when all the lava and tephra samples we have are already degassed? The answer lies in tiny bits of glass trapped inside of crystals. We call these bits of glass melt inclusions, because they represent the magma that was present at the time the crystals formed. Once a melt inclusion has been overgrown by a crystal, the volatiles are trapped inside and cannot escape*.
The concentration of sulfur measured in melt inclusions from Lakagígar ranges from ~1200 to 1800 parts per million (ppm). We can use the best estimate of 1675 ppm from Thordarson et al., 1996. In order to convert this concentration into an equivalent mass of S, we need to multiply by the total mass of lava erupted. We can assume a best estimate of 15 km3 of lava erupted.
*In detail this is not entirely true – volatiles can still diffuse out through the solid crystals at high temperatures – but for the purposes of our calculations we can assume that they remain perfectly entrapped.
1. Assuming a basalt density of 2750 kg/m3, what is the total mass of lava erupted in megatons (109 kg)? Watch your units! Not only do you need to convert kilograms to megatons, but you also need to convert cubic kilometers to cubic meters.
Now multiply the mass of lava you just calculated by 1675/106 to get the mass of sulfur in the magma prior to degassing.
2. What is the total mass of sulfur before degassing?
Now, just as with the CO2 in Coke experiment, you will also need to estimate the mass of sulfur after degassing, which is determined by measuring the concentration of sulfur in the degassed tephra and lava. The best estimate given by Thordarson et al. is 205 ppm.
3. Using the same total mass of lava you used above, calculate the mass of sulfur remaining after degassing.
The difference between these two masses is the mass of sulfur released to the atmosphere.
4. What is the total mass of sulfur released to the atmosphere (in megatons)?
One last thing. The sulfur released to the atmosphere is not pure elemental sulfur, it is mostly in the form of SO2 gas. In order to convert the mass of S into the equivalent mass of SO2, you will need to multiply by the mass ratio of SO2 to S. You can use any periodic table (I like WebElements [7]) to calculate the molar mass of SO2. Then simply divide this by the molar mass of S, and you have the mass ratio. Multiply by the total mass of sulfur released, and you’re done!
5. What is the total mass of SO2 released to the atmosphere (in megatons)?
6. Thordarson et al. calculated 122.1 megatons of SO2 released. How close did your calculation come to theirs?
Links
[1] https://commons.wikimedia.org/wiki/File%3ADiet_Coke_Mentos.jpg
[2] https://commons.wikimedia.org/wiki/File%3AShimadaK2007Sept09-MentosGeyser_DSC_3294%2B%2B.JPG
[3] https://www.e-education.psu.edu/rocco/sites/www.e-education.psu.edu.rocco/files/images/geosciences/Mentos_results.xlsx
[4] https://www.e-education.psu.edu/rocco/sites/www.e-education.psu.edu.rocco/files/Module%203%20Lab-corrected.docx
[5] http://www.chem1.com
[6] http://www.engineeringtoolbox.com/
[7] http://www.webelements.com