Essential Elements
You need only a two-liter bottle of soda and Mentos candies for this experiment. While the flavor of the candy doesn't matter, each kind of soda will react differently because ingredients will vary from brand to brand. You will achieve the best results with diet soda. Place the two-liter bottle of soda at the center of your experiment zone. Take four Mentos candies in your hand and quickly and carefully drop each one into the top of the open soda bottle. Then stand back. Within seconds, the geyser will spike from the bottle and soda will overflow from the bottle and onto the ground.
Scientific Principles
Soda is primarily water, sugar, flavoring and carbon dioxide and the gas suspended in the liquid cannot expand because of surface tension. When you drop candy into the bottle, it breaks the surface tension of the soda and the carbon dioxide starts to expand. With nowhere to go but up and out, the rapidly expanding gas forces the liquid to spew out of the bottle. This can cause a violent geyser to skyrocket into the air, or it can just cause an overflow. The reason Mentos candy works particularly well is because the candy has thousands of little pores on which the carbon dioxide can form its gas bubbles. Those pores help the gas expand more quickly. The result is made more extravagant because the candy sinks to the bottom of the bottle quickly, creating a rapid reaction. Diet soda is made with aspartame and potassium benzoate instead of sugar and this combination reduces the liquid's surface tension so that when the candy is introduced, the reaction is that much faster.
Safety and Cleaning
This is a fairly safe experiment, even though it does involve an explosion. An explosion of soda means there could be a lot of clean up and you will get sticky and wet, so perform this experiment outside. Equip yourself with goggles, a rain poncho and a hose or a bucket of water. Clean up the mess by dousing the sticky area with a bucket of water or by hosing it down. Another option is to try the experiment using different brands of soda or different flavors of Mentos.
Measuring
There are two ways of measuring this experiment, one is with measuring cups and the other with a yard stick. If you perform the experiment against a wall, you can easily measure the height of the geyser with a measuring stick. Alternatively, measuring the amount of soda left in the bottle will show how much soda was ejected during the experiment.