Why Does Shaking a Soda Make It Explode?

A shaken soda can erupt into a fizzy mess the moment it’s opened. The answer lies in how gases behave under pressure and what occurs when that balance is disrupted.

The Role Of Dissolved Carbon Dioxide

Carbonated beverages owe their fizz to dissolved carbon dioxide (CO₂), introduced under high pressure during manufacturing. This forces CO₂ into the liquid, forming carbonic acid (H₂CO₃) in equilibrium with free CO₂ molecules. The pressure inside a sealed container keeps most of the gas dissolved. When the container is opened, the pressure drops, allowing CO₂ to escape, forming bubbles that rise to the surface.

Henry’s Law states that the amount of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. In a sealed soda, high internal pressure maintains a significant concentration of dissolved CO₂. When the seal is broken, the external pressure drops, prompting CO₂ to escape. Even an undisturbed soda fizzes upon opening, though mildly compared to a shaken one.

Pressure And Bubble Dissipation

Inside a sealed soda container, the pressure keeps the liquid saturated with gas, preventing spontaneous bubble formation. High internal pressure suppresses nucleation, where gas molecules cluster to form bubbles. In an undisturbed soda, minimal bubbles form at microscopic imperfections on the container’s surface and usually dissolve before growing large enough to escape.

Shaking disrupts this balance. Agitation forces dissolved gas out of solution, creating numerous tiny bubbles that serve as nucleation sites, accelerating CO₂ release. Unlike the few bubbles in an undisturbed soda, these remain suspended and don’t have time to dissolve before the container is opened.

Opening the container after shaking causes a rapid pressure drop, allowing bubbles to expand and rise unchecked. Normally, internal pressure restrains bubble growth, but when the seal is broken, the external pressure drops to atmospheric levels, removing this constraint. The bubbles rapidly expand, pushing liquid out in a forceful eruption.

The Effect Of Agitation

Shaking alters CO₂ behavior within the liquid, setting up a more intense release upon opening. The disturbance forces gas out of solution, creating countless tiny bubbles that persist rather than dissipate. These bubbles increase the surface area for further CO₂ release, amplifying the effect.

Soda’s viscosity and the turbulence from shaking prevent bubbles from coalescing and rising efficiently, trapping them throughout the liquid. These bubbles act as nucleation points, accumulating additional CO₂ and making the soda increasingly unstable. Beverages with higher carbonation levels experience this effect more intensely, as more dissolved gas means greater bubble formation.

Temperature’s Role In Gas Release

Temperature affects how easily carbon dioxide escapes from the liquid. As temperature increases, gas solubility decreases. In a cold soda, CO₂ stays more tightly bound in the liquid, resulting in fewer bubbles. Warmer soda holds less dissolved gas, making it more prone to rapid gas release when disturbed.

A study in The Journal of Physical Chemistry B found that at 4°C (39°F), carbonated water retains nearly twice as much dissolved CO₂ as at 25°C (77°F). This means a cold soda, even when shaken, has fewer gas pockets available to drive an explosion. A warm soda, having already lost more CO₂, erupts more forcefully when opened.

Container Physics And The Release Mechanism

The material and design of a soda container affect how gas escapes upon opening. Whether in plastic bottles, aluminum cans, or glass bottles, each packaging type interacts differently with internal carbonation, influencing gas stability and bubble formation.

Plastic bottles are slightly flexible, allowing them to expand under pressure. This flexibility lets gas slowly diffuse through the plastic over time, reducing pressure compared to aluminum cans, which are rigid and maintain stable internal pressure, leading to a more forceful reaction when opened after shaking. Glass bottles, while also rigid, have a narrower neck, slightly restricting the rate of gas and liquid expulsion.

The opening mechanism also matters. Twist-off caps on plastic bottles release pressure more gradually, whereas a pull-tab on a can releases it instantly, sometimes causing a more violent eruption.