Carbon dioxide (CO2) readily dissolves in water (H2O). This phenomenon is evident in everyday life, from carbonated beverages to natural aquatic environments. Understanding the underlying chemistry explains why this interaction takes place.
The Molecular Makeup of Water and CO2
Water molecules possess a bent, or V-shaped, structure with an oxygen atom at the center bonded to two hydrogen atoms. Oxygen has a greater attraction for electrons than hydrogen, causing the shared electrons in the O-H bonds to be pulled closer to the oxygen. This creates a slight negative charge on the oxygen atom and slight positive charges on the hydrogen atoms, making water a polar molecule. Due to this polarity, water molecules can form attractions called hydrogen bonds with each other.
In contrast, a carbon dioxide molecule has a linear shape, with a carbon atom positioned between two oxygen atoms (O=C=O). Although each carbon-oxygen bond within the CO2 molecule is polar due to the difference in electron attraction between carbon and oxygen, the molecule’s symmetrical linear arrangement causes these individual bond polarities to cancel each other out. This results in carbon dioxide being an overall non-polar molecule.
The Chemistry of CO2 Dissolution
When carbon dioxide encounters water, the polar water molecules interact with the non-polar CO2. The partial charges on water molecules can induce temporary shifts in the electron distribution within the CO2 molecule. This creates weak electrical attractions, allowing water molecules to surround and attract the CO2 molecules.
A portion of the dissolved carbon dioxide then undergoes a chemical reaction with water. This reaction forms carbonic acid (H2CO3), represented by the equation CO2 + H2O ⇌ H2CO3. This process is significant because carbonic acid is what imparts a slightly acidic taste to carbonated beverages.
The formation of carbonic acid is a reversible reaction, meaning carbonic acid can also break down into carbon dioxide and water. Carbonic acid is considered a weak acid and, once formed, a small amount of it further dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3-). These dissociation reactions also exist in a state of dynamic equilibrium.
Factors Affecting CO2 Solubility
Two external conditions influence how much carbon dioxide can dissolve in water: temperature and pressure. These factors dictate the equilibrium between dissolved CO2 and gaseous CO2.
Temperature plays an inverse role in CO2 solubility; as water temperature increases, the amount of dissolved CO2 decreases. This is because higher temperatures provide gas molecules with more kinetic energy, making it easier for them to break free from the liquid solution and escape into the atmosphere. This principle is why a warm carbonated drink quickly loses its fizziness compared to a cold one.
Conversely, pressure has a direct relationship with CO2 solubility. Increasing the pressure of CO2 gas above a liquid forces more gas molecules into the solution. For instance, carbonated beverages are bottled under high CO2 pressure to maximize the amount of gas dissolved, creating the desired carbonation.
Where We See Dissolved CO2
The dissolution of carbon dioxide in water is a widespread phenomenon with many practical and natural implications. One familiar example is in carbonated beverages, where CO2 is intentionally dissolved under high pressure and low temperature to produce the characteristic bubbles and tangy flavor. When a bottle or can is opened, the external pressure drops, allowing the dissolved CO2 to escape as fizz.
In natural aquatic environments, dissolved CO2 serves as a carbon source for aquatic plants and algae. These organisms utilize dissolved carbon dioxide for photosynthesis, the process by which they convert light energy into chemical energy for growth.
Oceans play a role in absorbing atmospheric carbon dioxide. They act as a large reservoir, taking up approximately one-quarter of human-generated CO2 emissions. This natural absorption process helps regulate the concentration of CO2 in the atmosphere, influencing Earth’s climate system.