Why does a bottled carbonated beverage fizz when you take the cap off?

Le Chatelier's principle is a foundational concept in chemistry that explains how a system at equilibrium responds to changes in concentration, temperature, or pressure. This principle can be summarized as: if a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. In the context of carbonated beverages, when the cap of the bottle is removed, pressure is relieved. According to Le Chatelier's principle, the equilibrium of dissolved carbon dioxide will shift to counteract this decrease in pressure, resulting in more CO2 being released from the liquid in the form of bubbles.

This principle helps us understand numerous real-world chemical behaviors beyond fizzy drinks, including reactions in living organisms and industrial chemical synthesis.

Carbonated beverages get their fizz from dissolved carbon dioxide gas. Under the high pressure within a sealed container, CO2 is forced into solution with the beverage. The solubility of this gas is a direct result of the pressure applied, and the solution can hold more gas when pressure is higher. Once dissolved, carbon dioxide creates carbonic acid, which is responsible for the slight tang in the taste of carbonated drinks. The process of forcing CO2 into the beverage under pressure is carbonation and is key in making sodas and sparkling waters.

The interaction between the liquid and gas at a molecular level defines how carbonated a drink is and how long it stays fizzy after being opened.

The solubility of a gas in a liquid is heavily influenced by pressure, a relationship described by Henry's Law. According to Henry's Law, at a constant temperature, the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. This means that when the pressure over a liquid is increased, more gas will dissolve into the liquid; conversely, when pressure decreases, less gas will remain dissolved.

For carbonated drinks, this is why the CO2 stays dissolved under the high-pressure environment inside the sealed bottle and begins to escape—decreasing its solubility—once the bottle is opened and pressure drops.

An equilibrium shift occurs when the balance between two opposing processes is disrupted, causing the system to adjust to a new balance. In carbonated beverages, equilibrium exists between dissolved CO2 and gaseous CO2 in the space above the liquid. Removing the cap causes a drop in pressure, disrupting the equilibrium. The reaction shifts to release CO2 gas to re-establish equilibrium, which we observe as bubbling or fizzing.

This behavior is not only limited to soft drinks; it is a common phenomenon in various chemical systems where reactive species seek to maintain balance by adjusting concentrations in response to external changes, maintaining the system's overall stability.