FAQ: Mentos and Other Explosions

What causes the explosion when Mentos are dropped into Diet Coke?

Carbonated beverages have carbon dioxide (CO₂) in them. If you shake a can of Coke vigorously before you open it, the effect is similar to dropping Mentos in: the Coke shoots out in a narrow stream. In contrast, if you open an unshaken can of Coke, the CO₂ will leave gradually and quietly, and is easier to drink; the release of bubbles is so slow that it will be hours before the soda turns “flat.”

The CO₂ is dissolved in the Coke under pressure.  Gases, including CO₂, can dissolve into liquids much like sugar or salt can.  But the amount of gas that will dissolve into a liquid depends on the pressure that the gas and liquid are under.  At atmospheric pressure (about 15 psi), only a small amount of CO₂ will dissolve into Coke.  If you want more gas to dissolve, then you have to increase the pressure of the CO₂ until it is, say, two or three times atmospheric pressure. That’s why soda cans have to remain sealed: they contain CO₂ under pressure.  When you open the Coke container, the pressure is reduced (to 15 psi), and the CO₂ immediately begins to leave the Coke. 

That release can happen at the surface of the liquid, or, the CO₂ can form bubbles that will rise to the top of the liquid and escape.  However, for CO₂ to form a bubble at depth in a bottle of Coke, it takes energy to shove the water molecules aside to make a “hole” in the liquid that the gas can move into.  If all the CO₂ molecules have the same energy, they can’t very easily get together to form a hole at a particular location in the liquid.

To understand how energy is involved, open two cans of soda side by side; one that is cold from your refrigerator, and one that is warm. The warm one will release bubbles of CO₂ much faster, because a warm gas is more energetic.

Whether the soda is warm or cold, the process of forming bubbles occurs slowly.  However, if there are sharp edges or fine particles in the liquid, these have surfaces that allow the CO₂ molecules to start bubble formation more easily (these are called nucleation points).  Mentos tablets contain thousands of these nucleation points and when dropped into Coke, they allow the bubbles to form almost instantaneously.

To understand how the surface of the Mento causes the CO₂ bubbles to nucleate, think about how rock candy is made. A string or a stick is immersed into a supersaturated mixture of sugar and water, and crystals of sugar nucleate around the stick. Without immersing the stick, the crystals will grow eventually, but the stick speeds up the process by providing a surface for nucleation.

This pressure effect of gases and liquids also has a more dangerous aspect as well: when scuba divers go to great depths in the ocean, they are breathing air, or specialized gas mixtures, at four or more times atmospheric pressure.  Because of this, nitrogen dissolves into their blood stream in much higher amounts than would happen at the ocean surface.  If they come back to the surface too rapidly, the nitrogen dissolved in their blood and tissues begins to bubble out, causing a condition called the “bends.”  If they rise to the surface more slowly, these gases will gradually be flushed from their blood and they’ll suffer no ill effects from the change in pressure.

What causes the explosion in the garbage can with tennis balls?

The exploding water in the garbage can demonstrates what happens when a gas, which is confined by pressure, is released suddenly: it expands very rapidly, blowing out everything in its way. The gas used in this experiment is liquid nitrogen, which was carefully anchored beneath the water. Do not try this at home! Liquid nitrogen should only be handled by experts.

Watch the slow-motion video carefully, and observe how the Brute garbage can (the blue seamless one) deforms during the explosion. The multi-colored garbage can does not fare as well.

What does this have to do with geology?

Scientists are interested in the details of how volcanoes erupt, so they analyze temperature, pressure, and the chemistry of magma. Some volcanologists study the way gas bubbles nucleate and expand, using laboratory models to simulate magma processes. Gases are dissolved in magma at pressures many thousands of times atmospheric pressure.  The rate that these gases can escape the magma solution makes the difference between the relatively quiescent eruptions of Kīlauea, and the much more explosive eruptions of Mount Vesuvius.  The experiments shown in the video were performed at UH Hilo’s Geology Department, in connection with classes in volcanology.