Carbon dioxide (CO2) and water (H2O) are abundant molecules on Earth. Their interaction can lead to a series of chemical changes with far-reaching implications. This reaction is fundamental to various natural processes and human activities, influencing everything from the chemistry of oceans to the fizz in beverages and the regulation of bodily functions.
Forming Carbonic Acid
When carbon dioxide gas dissolves in water, a portion of it undergoes a chemical reaction to form carbonic acid (H2CO3). The reaction is represented by the reversible chemical equation: CO2 + H2O ⇌ H2CO3. This indicates that carbonic acid can also break down back into carbon dioxide and water.
The formation of carbonic acid is a relatively slow process without the presence of catalysts. Even when CO2 dissolves in water, much of it remains as dissolved carbon dioxide rather than immediately converting to carbonic acid. This equilibrium signifies a constant interplay where molecules are both forming and breaking apart, influencing the overall chemical state of the solution.
What Happens Next
Carbonic acid (H2CO3) is an unstable compound that readily dissociates in water. This dissociation occurs in a stepwise manner, releasing hydrogen ions (H+) into the solution. In the first step, carbonic acid loses one hydrogen ion to form a bicarbonate ion (HCO3-) and a hydrogen ion, often represented as a hydronium ion (H3O+) in water.
The bicarbonate ion can then further dissociate, losing another hydrogen ion to become a carbonate ion (CO3^2-). The release of these hydrogen ions is significant because they directly influence the acidity of the solution. An increase in hydrogen ion concentration leads to a decrease in pH, making the solution more acidic.
Real-World Impact
The reaction between carbon dioxide and water, along with the subsequent dissociation of carbonic acid, profoundly impacts several real-world systems.
Ocean Acidification
One prominent example is ocean acidification, where increased atmospheric carbon dioxide dissolves into the oceans. This process forms carbonic acid, which then releases hydrogen ions, leading to a measurable decrease in ocean pH. Since the Industrial Revolution, the average pH of the ocean surface has declined, impacting marine calcifying organisms like shellfish and corals that rely on calcium carbonate to build their shells and skeletons.
Carbonated Beverages
Another familiar application is in carbonated beverages, such as sodas and sparkling water. Here, carbon dioxide is dissolved into water under high pressure. This dissolved CO2 creates carbonic acid, which contributes to the slightly sour taste and the characteristic fizz. When a container is opened, the pressure is released, causing the dissolved carbon dioxide to escape as bubbles, making the drink go “flat” over time.
Human Body pH Regulation
Within the human body, the bicarbonate buffer system plays a crucial function in maintaining stable blood pH. Carbon dioxide produced by cellular metabolism reacts with water to form carbonic acid, which then dissociates into bicarbonate ions and hydrogen ions. This system helps to neutralize excess acids or bases, ensuring that the blood’s pH remains within a narrow, healthy range, which is necessary for various bodily processes and enzyme functions.
How Conditions Affect the Reaction
Several conditions can influence the extent and rate at which carbon dioxide reacts with water.
Temperature
Temperature is a significant factor, as carbon dioxide is more soluble in colder water than in warmer water. This is why carbonated beverages tend to lose their fizz more quickly at room temperature compared to when they are kept chilled. As water temperature increases, the solubility of CO2 decreases, causing it to escape from the solution.
Pressure
Pressure also plays a direct role in the solubility of carbon dioxide in water. Higher pressure increases the amount of CO2 that can dissolve, which is why carbonated drinks are bottled under pressure to keep the gas dissolved. When the pressure is reduced, such as by opening a bottle, the solubility decreases, and the CO2 escapes, creating bubbles. This relationship between pressure and gas solubility is described by Henry’s Law.
Catalysts
Biological systems often employ catalysts to speed up this reaction. Carbonic anhydrase is an enzyme found in various organisms, including humans, that significantly accelerates the interconversion of carbon dioxide and water into carbonic acid, and its dissociation products. This enzyme is particularly important in red blood cells for the efficient transport of carbon dioxide from tissues to the lungs for exhalation. It ensures that the hydration and dehydration of CO2 occur rapidly enough to support physiological needs.