Watching Mentos chewy candies produce shooting geysers when placed into a bottle of soda makes for an educational science project. In addition to performing the demonstration, you can explain the scientific factors that cause the shooting fountains. For the safety of your children or students, it is recommended that you conduct this demonstration outdoors. In addition, you may ask that children or students wear safety goggles.
The heavily carbonated sodas release carbon dioxide when opened. Carbon dioxide produces those bubbles you see when sodas fizz. These bubbles are a key component for allowing sodas to shoot out of the bottle when Mentos are dropped in. Diet sodas contain more carbon dioxide, which, in turn, produces more bubbles. You will therefore see a stronger geyser when you use a bottle of diet soda in your demonstration.
Instead of expanding, like normal gasses do, water molecules in the soda all bond with each other, tightening the surface tension around the carbon dioxide as the soda remains pressurised inside a sealed bottle. This is why you do not see fizz in an unopened, unshaken bottle of soda. Once the bottle is opened, however, the carbon dioxide is allowed to expand, causing tiny bubbles, or fizz, to form.
The surface area of each piece of Mentos is uneven, with many minuscule holes we cannot see. As soon as the candies touch the soda, they start dissolving in the liquid. The molecules contained in the candy force the carbon dioxide molecules of the soda apart. This upsets and quickly breaks the strong surface tension previously created by the water molecules. The more pieces of candy you add at once, the stronger the disruption. The force from this quick reaction instantly sends the soda shooting out in a strong stream.
For older children and students, you may point out that the reaction which takes place is a physical, not chemical one. This means that after the demonstration has been conducted, both the soda and Mentos remain in their original carbonated liquid and solid states, respectively. (In chemical reactions, substances are transformed into new substances with an entirely different set of physical, chemical and molecular properties from their original states.)
The temperature of the soda also affects how high the soda fountain will be when Mentos are dropped into the bottle. Adding the candies into room temperature or warmed bottle of soda will allow the soda to shoot higher as it explodes from the bottle. Chilled sodas, on the other hand, produce a much smaller explosion. For general safety purposes, you may want to chill the soda bottle before performing the demonstration in front of younger children.
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