Why Do Coke and Mentos React?
They don’t, at least not chemically. What we see when a Mentos candy is dropped into Coke—a fountain of soda erupting from the bottle—is the result of a physical reaction. And though this pair is the most famous for this effervescence, the rule of physics that causes it works with plenty of other carbonated drinks and rough candies, but with variable success.
Let’s dive into a bottle of Coke to understand the science behind this “soda fountain.” At the molecular level, Coca-Cola, or Coke, is mostly carbonated water with additives such as sugar (or artificial sweeteners), caffeine, and flavorings. Adding carbon dioxide (CO2) makes Coke fizzy, enhances its tangy taste, and helps preserve it by preventing spoilage. The beverage is bottled under high CO2 pressure to increase the gas’s solubility in water. This process is governed by Henry’s Law, which states that a dissolved gas’s concentration is directly proportional to its partial pressure above the liquid—the higher the pressure, the greater the amount of gas dissolved. In a sealed Coke bottle, the CO2 maintains a dynamic solubility equilibrium, where the rate of gas dissolving into the liquid equals the rate of gas escaping into the empty headspace above the liquid. Opening the cap of the bottle shatters this balance. As the pressurized CO2 in the headspace escapes, the partial pressure over the liquid plummets. To restore equilibrium, the dissolved CO2 rapidly rushes out of the solution as gas bubbles, creating the fizz most of us are familiar with.
Forming bubbles in the beverage, however, is difficult because Coke is mostly water, and water molecules strongly stick together because of high surface tension and cohesive forces. For bubbles to form, these water molecules must be moved apart to create gaps that the gas can occupy. To break through the water’s tight molecular network, the escaping CO2 relies on nucleation sites—microscopic rough spots, impurities, or foreign objects that disrupt the arrangement of water molecules and allow gas molecules to easily cluster. The congregating gas molecules merge into larger bubbles at these sites. As each bubble traps more gas, its buoyancy increases until the bubble floats to the surface.
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- A 2-liter (0.5-gallon) bottle of Coke contains 15 grams (0.5 ounce) of dissolved CO2, which bubbles out to occupy a volume of 8 liters (2.1 gallons) within seconds, causing the soda explosion.
- As of 2026 the highest soda geyser produced was 10.4 meters (34.1 feet) high.
- Simultaneously consuming Coke and Mentos does not produce the characteristic geyser effect in the human stomach because the candy largely dissolves during ingestion, and the beverage loses much of its dissolved CO2 during consumption, preventing the formation of the specific conditions required to trigger a rapid explosive reaction.
Mentos candies, despite their smooth appearance, have numerous small cracks, crevices, pores, and scratches, which act as nucleation sites. When a candy is dropped into Coke, the dissolved CO2 diffuses directly into these pockets on the candy’s surface. As the gas rapidly accumulates within these tiny crevices, the bubbles expand almost instantaneously and push the beverage violently out of the bottle to create a soda fountain, also called a geyser. Physicists have coined a term for this phenomenon: nucleation-induced soda degassing.
Conditions for optimal sprays
Because the solubility of CO2 is inversely proportional to temperature—the higher the temperature, the lower the solubility—warmer soda releases gas more rapidly. Room-temperature bottles therefore produce much higher fountains than chilled ones. Diet Coke yields even taller eruptions than regular Coke. Earlier researchers suggested that the lack of sugar and the presence of additives (such as the sweetener aspartame and the preservative potassium benzoate) reduced surface tension in Diet Coke, launching the liquid farther. However, a 2017 study observed that these dissolved solutes likely work by preventing tiny bubbles from merging. By keeping the bubbles small, the solutes may enable the total surface area between the liquid and the gas to increase drastically. This would allow the dissolved CO2 to diffuse and escape far more rapidly, creating a faster, more explosive eruption.
The number and type of Mentos candies used also influence spray height. According to some experimenters, fewer than five candies tend to produce shorter fountains, and they suggest using seven candies in a 2-liter (0.5-gallon) Coke bottle for the greatest geyser height. Strawberry-flavored candies, which contain citric acid, produce taller fountains than mint-flavored varieties that lack the additive. This variation may occur because citric acid inhibits bubble coalescence, similar to the additives in Diet Coke.