Co-Authored by Dean J. Campbell*, Thomas Kahila*, and Cassidy Kraft* *Bradley University, Peoria, Illinois
Many people are familiar with the demonstration that use Mentos candies to produce fountains from carbonated beverages (soda fountains).1-5 In most cases, the soda is initially in a plastic bottle rather than an aluminum can, since Mentos candies fit into the mouths of plastic soda bottles and not into the mouths of aluminum cans. An inverted funnel-like shape can be found near the mouths of plastic bottles that helps direct foam fountains upward, and the screw-like threads at the mouth can be used to connect various fountain-launching devices. In contrast, aluminum cans have flat tops that do not help funnel fountains upward and do not have a way to attach existing devices for launching soda fountains. However, aluminum cans still contain sufficient soda to produce fountains. Additionally, unopened aluminum cans appear to lose their carbon dioxide less quickly than unopened plastic bottles, meaning that soda could stay more carbonated for a longer time in a can. Mentos candies are also not necessary to produce soda fountains, as other objects can cause a soda fountain to form. Both TicTac candies and reusable rusty iron metal spheres can be used, and both fit into the mouth of an aluminum can. While commercially available attachments for soda fountains exist for plastic bottles,6 none have been found for soda cans. Inexpensive, easily accessible materials can be used as launchers on aluminum cans of carbonated beverages to produce soda fountains and even a soda fountain “rocket”.
In the classic Mentos/Diet Coke soda fountain, the surface of the Mentos nucleates the formation of carbon dioxide gas bubbles in the supersaturated carbonated beverage.1-5 The textured surfaces of the Mentos act as heterogeneous catalysts for the reaction CO2 (g) → CO2 (l). The fountains produced by this widely popular demonstration can vary widely in height, sometimes producing fountains that do not fit within the low ceilings of many classrooms. As a result, research has been done to study factors that influence fountain height, including aspects of the Mentos (or related objects) and aspects of the soda such as density, viscosity, and surface tension. 1-5,7 Additionally, the fountain launchers can be designed to produce taller or shorter fountains as described below.
There were several criteria to be considered when designing a soda fountain launcher for use on an aluminum can:
- The launch tube should be aligned vertically with the mouth of the can, and the inner diameter of the tube should be smaller than the mouth of the can. The tube should hold and deliver the fountain nucleating objects downward into the soda, and allow the soda fountain foam to be directed upward.
- The holder for the tube must be able to keep the tube oriented vertically and provide a strong watertight seal with the mouth of the can. If the seal material holding the tube to the can opening is not sturdy enough to handle the pressure and water content of the soda geyser (e.g., too soft or water soluble) it will not repeatedly hold the tube in place properly for the fountain to proceed as intended.
- There must be a method to hold the fountain nucleating objects in the tube that allows the objects to be released all at the proper time without interfering with the soda fountain. Many commercially-available fountain launchers have a retractable release pin that supports Mentos in the launch tube.6 As the pin is pulled away, the Mentos fall into the soda. For a home-built tube with sufficiently soft and thin walls (e.g., a drinking straw), a stick pin can be inserted through the tube to support the nucleating objects. The tiny pin hole in the tube leaks a negligible amount of soda when the fountain is produced. Alternatively, ferromagnetic nucleating objects such as iron powder7 or iron spheres can be held in in place using a magnet until launch.
- The materials used should be readily accessible, cost-effective, and reusable.
In one set of experiments, various types of tubes were tested by placing small rusty iron spheres (BBs) in the vertically oriented tube and securing them by placing a small ceramic magnet against the outside of the tube. The tube was then carefully placed slightly inside the opening of the can and the rest of the can opening was sealed with the material used to hold the tube upright. The spheres were then dropped into 355 mL cans of soda for each trial by removing the magnet from the side of the tube and the resulting fountain was observed. The best-performing tubes in the early trials were bubble tea straws and glass tubes. Both types of tubes had a smooth, rigid interior with a uniform diameter that readily released the spheres when the magnet was removed. Poorly performing tubes included a modified medicine spoon with a hollow hexagonal plastic handle (spheres appeared to become wedged in the corners); a 10 mL syringe tube (spheres became stuck on an inner lip); and cross-linked polyethylene (PEX) tubing (spheres appeared to catch on the rough, soft interior of a tube that was bent out of a uniform profile).
All of the tubes that were tested needed to be able to stand upright in the can. If the external diameter of a tube was narrower than the mouth of a can, the tube end could be effectively widened by wrapping it with duct tape. The widened tube could then be wedged vertically into the mouth of the can. Despite widening the outer diameter of the tube, this approach did not actually seal the mouth of the can. One of the best-performing can seals/tube holders was made from a type of modeling clay (e.g. FIMO Kids Oven-Hardening Modeling Clay, STAEDTLER Group, Germany). The seal held its shape when molded around the tubes and remained usable when exposed to open air for a significant amount of time. This clay also did not absorb water from the soda, allowing it to be reused both immediately and over a long period of time. Other putty-like materials studied were less successful.
Another excellent holder for the tubes was a reusable plastic can cap and built-in soda straw (Jokari US Inc., Carrollton, TX), in which the plastic straw was removed and replaced with a wider glass tube which was inserted in the hole of the cap. The tube was supported upright and sealed by the plastic of the cap. Figure 1 shows this can cap and glass tube assembly in use with rusty BBs to make a soda can fountain.
Figure 1. (LEFT) View of the assembly showing the plastic cap, glass tube, magnet, and 35 g of rusty iron BBs atop the aluminum can filled with soda. (RIGHT) Fountain produced by the addition of 35 g rusty iron BBs from the assembly pictured at left. The marks on the stick represent 10 cm intervals.
Figure 2 shows the reusable plastic can cap and built-in soda straw modified in different way to drop Tic Tac candies into a can of soda. Here a stick pin is added to the plastic tube to support the candies before launch, the gap between the plastic tube and the horizontal can cap is sealed with epoxy adhesive, and the air vent in the horizontal can cap is sealed with electrical tape. Finally, a circle of foam padding that is slightly smaller than the diameter of the top of the can is glued to the underside of the horizontal can cap, filling in much of the space between the can and cap so the soda foam is directed into the tube during fountaining. Video 1 shows the use of a launcher similar to the one shown in Figure 2. For a color effect, blue food coloring was added to the soda before the launcher was attached to the can, so the soda fountaining out of the tube had a blue tint.
Figure 2. Generalized scheme for conversion of the reusable plastic can cap into a soda fountain launcher. (LEFT) Top view of launcher, showing stick pin pushed through the plastic tube supporting the Tic Tac candies, the gap between the plastic tube and the horizontal can cap sealed with epoxy resin, and the air vent in the horizontal can cap is sealed with electrical tape. (RIGHT) View of the underside of the launcher showing the flexible foam pad used to fill the space between the top of the can and the can cap.
Video 1. Dropping Tic Tac candies into a soda can. ChemDemos YouTube Channel (accessed 9/15/2021)
Another intriguing variation on a soda fountain launcher that attaches to an aluminum can is based on reusable plastic six-pack aluminum can holders. These do not seal to the top of the can as well as the reusable plastic can cap and built-in soda straw, but might be readily available for free from local beverage retailers. Figure 3 shows a generalized scheme for conversion of the reusable six-pack can holder and plastic tube and foam pad into a soda fountain launcher using the following steps:
Step 1 - Cut one of the circular pieces off of a soda can six-pack holder (top left panel).
Step 2 - Drill a hole in order to place the plastic tube through the can holder piece (top right panel). For larger holes, a pilot hole made using a smaller bit might be advisable.
Step 3 - Place the plastic tube through the hole in the can holder piece. Cut a small hole in a piece of foam pad, place it under the can holder piece, and place the plastic tube through the hole in the foam. In the version shown in the bottom left panel of Figure 3, the plastic foam pad is a flexible sheet that is bigger than the can holder piece or the top of the can. Although the foam pad is designed to seal across the entire edge where the can meets the can holder, the foam pad made it difficult to get the holder to attach to the can. An alternative way to use the foam pad is to cut it to slightly smaller than the diameter of the top of the can, enabling the can holder to attach to the can while still mostly filling in the space between the can and holder. It should be noted that for the launchers based on the reusable six-pack holders it is very difficult to completely stop the soda foam from leaking out from between the can and the holder (although the plastic foam pads help significantly). Since the soda fountains produce a bit of a mess anyway, this leakage should not present a problem.
Step 4 – Glue the foam pad to the can holder (e.g., with silicone caulk) and the plastic tube to the can holder (e.g., with epoxy adhesive). The reusable can holder version of the soda can launcher is shown assembled in Figure 3 lower right corner.
Figure 3. Generalized scheme for conversion of soda can holder, plastic tube, and foad pad into a soda fountain launcher. (TOP LEFT) Soda can six-pack holder. (TOP RIGHT) Drilling a hole for placement of the plastic tube. (BOTTOM LEFT) General placement of drilled soda can holder and foam pad (with hole) on the plastic tube. (BOTTOM RIGHT) A version of the soda can launcher fastened to the top of a soda can.
The version of the fountain launcher shown in Figure 3 used a bubble tea straw as a plastic tube, however, other straws can be used as well. Figure 4 shows the reusable six-pack holder based fountain launcher using a rather narrow straw. It is anticipated that the ability to vary the diameter of the tube used in these launchers provides a way to control the height of the soda fountain, with narrower straws producing higher fountains. Another variation is to use the soda fountain to propel a sort of “rocket,” made from a larger straw with an end closed with tape. To use the rocket, the narrow straw is loaded with rusty BBs and held in place with a magnet, then the larger straw with the closed end up is placed over the smaller straw. The magnet is pulled away from the launcher, the BBs drop into the soda, and the resulting fountain pushes the rocket skyward. This rocket configuration is similar to cutting the end off of a soda straw wrapper and blowing the wrapper off of the straw, and also more carefully designed rockets launched by air from a straw.8 Video 2 shows the BB-initiated soda fountain rocket in use on a can of seltzer water.
Figure 4. Soda fountain launcher made from a portion of a soda can six-pack holder, a soda straw, and a round foam pad. Rusty BBs are held in the soda straw by attraction to the magnet against the outside of the straw. (LEFT) View from above. (MIDDLE) View of the underside of the launcher showing the straw placed in the hole in the foam pad. (RIGHT) A “rocket” made from a soda straw closed at the top with tape has been placed over the narrower straw containing the rusty BBs.
Video 2. Soda can fountain with straw rocket (slow motion). ChemDemos YouTube Channel (accessed 9/15/2021)
To conclude, there are several launcher options that can be used in order to produce carbonated beverage fountains from aluminum cans. Various launchers can be constructed and compared in classroom projects to explore what most influences fountain height (e.g., the diameter of the launcher tube and the number of fountain-initiating objects in the tube). The fountain launchers can be assembled from inexpensive parts, some of which, like straws and flexible plastic foam, are often destined for the trash. In this case, reuse of these objects can be thought of as an environmentally responsible move. Along similar lines, this demonstration and comparison between aluminum can and plastic bottle based fountains could serve as a springboard for further discussions of environmental chemistry topics. Green chemistry focuses on reducing the environmental impact of the field of chemistry and is based around twelve core principles. The specific principle from the Twelve Principles of Green Chemistry that might be best highlighted in comparing aluminum cans and plastic bottles is Design for Energy Efficiency.9 Production of aluminum metal from its ore is notoriously energy intensive, but aluminum cans have greater recycling rates than plastic bottles in some areas. Other considerations to help drive discussions are that although plastic bottles can be recapped more easily than aluminum cans, beverages in aluminum cans pack more efficiently and are easier to cool than in plastic bottles.10
The final idea for a soda fountain launcher is again based on the reuse of objects that are typically used once and then discarded, namely a kit used to fill water balloons. There is a water balloon kit where all of the uninflated balloons are attached to an apparatus that uses an assembly of narrow tubes to connect the balloons to an outdoor faucet (e.g. BunchOBalloons, ZURU Inc., Kowloon, Hong Kong). Simply screw the assembly to the faucet, turn on the faucet, and many water balloons all fill simultaneously and can be slid off of the assembly when they are full. After this use, the assembly is meant to be recycled or discarded. However, it can also be reused in the context of soda fountains. The threaded faucet connector at the ends of the tubes on the water balloon filler assembly fits the threads at the top of a commercially available Mentos launcher used to make soda fountains from plastic bottles.6 Video 3 shows the shower-like soda fountain produced by the water balloon assembly connected to a typical soda fountain setup containing a commercial soda fountain launcher and a small bottle of soda. It should be noted that the soda had been the bottle for a long time before opening and therefore most likely did not provide the peak amount of carbonation possible. However, a nice soda fountain was still produced.
Video 3. Mentos soda fountain through a water balloon filling assembly. ChemDemos YouTube Channel (accessed 9/15/2021)
Do not ingest any of the beverages or candies used in the demonstrations in case of accidental contamination from other chemical sources. Personal protective equipment such as goggles should be used when working with fountains, even of carbonated beverages. The objects used as catalysts for the fountain reaction can be launched back up through the tube, becoming small projectiles. Consider what people might say if one got a BB in their eye! Using pigmented beverages risks staining surfaces, as one of the authors once did to a ceiling tile. Using sugary beverages can leave behind sticky surfaces. Therefore, colorless, sugar-free carbonated beverages are highly recommended. Tarps or small plastic swimming pools are recommended to help contain spills and splashes. Hands should always be washed after completing a demonstration.
This work was supported by Bradley University and the Mund-Lagowski Department of Chemistry and Biochemistry with additional support from the Illinois Heartland Section of the American Chemical Society and the Illinois Space Grant Consortium.
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For Laboratory Work: Please refer to the ACS Guidelines for Chemical Laboratory Safety in Secondary Schools (2016).
For Demonstrations: Please refer to the ACS Division of Chemical Education Safety Guidelines for Chemical Demonstrations.
Other Safety resources
RAMP: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies