The Science of Carbonation
Many believe shaking an unopened soda makes it flat before opening. However, shaking an unopened soda does not make it flat while sealed, but it sets the stage for a messy eruption upon opening. This phenomenon is rooted in the physical properties of carbonated beverages and dissolved gases.
Soda gets its fizz from dissolved carbon dioxide (CO2) gas. Manufacturers infuse the liquid with CO2 under high pressure in a sealed container. This forces gas molecules into the liquid, forming carbonic acid, which contributes to the beverage’s taste and effervescence.
Inside the sealed can, an equilibrium exists between the dissolved CO2 in the liquid and the CO2 gas occupying the headspace. This equilibrium is maintained by the constant pressure exerted by the gas in the headspace. This pressure keeps CO2 molecules trapped within the liquid, preventing escape and maintaining carbonation.
What Shaking Does Inside the Can
When an unopened soda is shaken, the movement agitates the liquid and dissolved carbon dioxide. This agitation disrupts the equilibrium, causing dissolved CO2 to rapidly escape the liquid and form gas bubbles. These gas bubbles quickly rise into the container’s headspace.
As CO2 gas accumulates in this confined space, pressure inside the can builds. Shaking provides nucleation sites, like microscopic imperfections, where dissolved CO2 more easily transitions into a gaseous state. This converts dissolved CO2 to a gaseous state in the headspace, increasing internal pressure, but does not remove it from the can.
The Explosive Opening
Opening a shaken soda can instantly releases the built-up internal pressure. The sudden drop in pressure causes trapped CO2 gas to expand dramatically. This expansion forces liquid and foam out, resulting in an eruptive spray.
The rapid escape of gas quickly depletes carbonation as CO2 rushes out. The soda was not flat before opening; it lost its fizz rapidly at depressurization. The sudden pressure release causes remaining dissolved CO2 to rapidly bubble out, creating a foamy eruption.
How Soda Truly Loses Its Fizz
Soda loses its fizz once its seal is broken and it’s exposed to the atmosphere. With pressure released, dissolved carbon dioxide naturally escapes the liquid into the air. This occurs gradually as CO2 molecules move from higher concentration (the soda) to lower concentration (the atmosphere).
Warmer temperatures (reducing gas solubility) and increased surface area (like pouring into a wide glass) accelerate this degassing. The absence of sealed pressure allows CO2 to continuously leave the liquid until carbonation is gone. Thus, carbonation loss is a slow, continuous process that begins only after a soda is opened, not when shaken while sealed.