When carbon dioxide gas ($\text{CO}_2$) dissolves in water ($\text{H}_2\text{O}$), a common and reversible chemical reaction occurs. This interaction is fundamental to processes ranging from global ocean chemistry to human blood function. The combination of these two molecules results in the formation of an acid, immediately changing the nature of the water solution. This chemical event drives numerous phenomena, demonstrating how a subtle molecular change can have large-scale consequences.
The Chemistry of Carbonic Acid Formation
The initial step involves the direct combination of carbon dioxide and water molecules. The $\text{CO}_2$ molecule is incorporated into the water structure to produce carbonic acid ($\text{H}_2\text{CO}_3$). The reaction is represented by the reversible equation: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3$.
Carbonic acid is classified as a weak acid, meaning it does not fully ionize in solution. $\text{H}_2\text{CO}_3$ is inherently unstable when dissolved in water. It does not remain intact for long, setting the stage for the next chemical event that determines the properties of the resulting water mixture.
Dissociation and the Change in Acidity
The unstable carbonic acid immediately begins to dissociate in the water solution. This dissociation is the second step, fundamentally altering the chemical nature of the water. The $\text{H}_2\text{CO}_3$ molecule splits into a bicarbonate ion ($\text{HCO}_3^-$) and a free hydrogen ion ($\text{H}^+$).
This dissociation is the source of free hydrogen ions, which increases the solution’s acidity. The reaction is represented by the equation: $\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$. The concentration of these $\text{H}^+$ ions is measured using the $\text{pH}$ scale. Since a higher concentration of hydrogen ions corresponds to a lower $\text{pH}$ value, the release of $\text{H}^+$ ions causes the $\text{pH}$ of the water to decrease.
Global Impact in Ocean Acidification
The reaction between carbon dioxide and water has significant global repercussions, notably in ocean acidification. Since the Industrial Revolution, human activities have dramatically increased atmospheric $\text{CO}_2$. The ocean has absorbed approximately 30% of this excess gas, shifting the chemical equilibrium in seawater and leading to a rise in carbonic acid formation.
The resulting increase in hydrogen ions lowers the ocean’s average surface $\text{pH}$, which has fallen by about 0.1 units since the pre-industrial era. This shift represents a substantial increase in ocean acidity due to the logarithmic nature of the $\text{pH}$ scale. The $\text{H}^+$ ions also react with carbonate ions ($\text{CO}_3^{2-}$), which are the building blocks for shells and skeletons.
The reduction in available carbonate ions creates severe challenges for calcifying organisms. These species rely on carbonate and calcium to build and maintain their calcium carbonate structures. Examples of affected organisms include:
- Corals
- Oysters
- Clams
- Pteropods (sea snails)
As the water becomes more acidic, it is increasingly difficult for them to grow shells, and existing shells can begin to dissolve. This disruption threatens the entire marine food web, as many calcifying species form the base of the ocean food chain.
Applications in Human Biology and Carbonated Drinks
The carbon dioxide-water reaction is a necessary process in human physiology and commercial products. In the human body, this reaction forms the basis of the bicarbonate buffering system, the primary mechanism for maintaining stable blood $\text{pH}$. When $\text{CO}_2$ is produced as a metabolic waste product, it reacts with water to form carbonic acid, which then dissociates into bicarbonate and hydrogen ions.
The bicarbonate ions ($\text{HCO}_3^-$) are transported to the lungs, where the process reverses to form $\text{CO}_2$ that is exhaled. Bicarbonate acts as a weak base, neutralizing excess acids in the bloodstream. This prevents blood $\text{pH}$ from dropping below the healthy range of 7.35 to 7.45, regulating acid-base balance while managing $\text{CO}_2$ transport.
The reaction is intentionally manipulated for commercial purposes in carbonated beverages. To create the characteristic “fizz,” $\text{CO}_2$ gas is dissolved into water under high pressure. This dissolution creates a supersaturated solution where a small amount of gas reacts to form carbonic acid, contributing a slightly tangy flavor. When the container is opened, the pressure is released, causing the equilibrium to reverse, and the dissolved $\text{CO}_2$ escapes as bubbles.