Water is often called the universal solvent because of its ability to dissolve more substances than any other liquid. The scientific answer to whether gases mix with water is found in the concept of solubility. Solubility quantifies the maximum amount of one substance that can integrate into another. Understanding this process requires examining the molecular interactions that govern how gas molecules integrate into the liquid structure.
The Scientific Answer: Solubility
Gases dissolve in water, but the extent depends on the specific chemical properties of the gas. Gases like oxygen and nitrogen are sparingly soluble, meaning only a small quantity integrates into the liquid phase. Conversely, gases such as ammonia and carbon dioxide exhibit high solubility, dissolving readily in greater concentrations. This variability results from the forces between the gas and water molecules.
The Mechanics of Mixing
The mechanics of gas dissolution depend on the intermolecular forces exerted by the water molecule. Water is a highly polar molecule, having a slight positive charge near the hydrogen atoms and a negative charge near the oxygen atom. These strong electrostatic forces dictate whether an incoming gas molecule is attracted to or repelled by the surrounding liquid.
When a highly polar gas, such as ammonia, approaches the water, it forms strong attractive forces with the water molecules, allowing it to easily break apart the liquid structure and dissolve. The polarity of water pulls the gas molecules into solution. The resulting mixture is energetically favorable, meaning the gas is stabilized within the liquid environment.
In contrast, non-polar gases like oxygen or methane lack this charge separation and cannot form strong attractive bonds with the water. These non-polar molecules occupy the tiny interstitial spaces between the closely packed water molecules. Water molecules must temporarily rearrange themselves around the non-polar gas, creating a slight, unfavorable energy state for the liquid. This lack of attraction means that non-polar gases are less stable and attempt to escape back into the atmosphere.
External Factors Governing Solubility
Beyond the inherent chemical properties of the gas and water, two external physical variables control the amount of gas that can dissolve. The first is pressure, described by Henry’s Law, which states that the solubility of a gas is directly proportional to the partial pressure of that gas above the liquid. Increasing the pressure forces more gas molecules into the liquid phase, thereby increasing the concentration of the dissolved gas.
This principle is applied commercially in the bottling of carbonated beverages, where carbon dioxide gas is sealed under high pressure to maximize its dissolution in the drink. Once the bottle is opened, the pressure above the liquid drops dramatically, and the gas escapes rapidly from the solution. The second factor is temperature, which exhibits an inverse relationship with gas solubility in liquids.
As the temperature of the water increases, the kinetic energy of the water molecules also increases, causing them to move faster and spread further apart. This increased molecular motion allows the dissolved gas molecules to escape the solution more easily and return to the atmosphere. This temperature dependence has significant implications for aquatic environments, as warmer waters hold less dissolved oxygen, which can stress fish and other organisms that rely on atmospheric gas for respiration.
Everyday Examples of Dissolved Gas
The dissolution of gases in water is responsible for many common phenomena and environmental processes observed daily. The fizz in a soda or sparkling water is the most familiar example, where carbon dioxide is forced into the liquid under high pressure to create carbonic acid. This process demonstrates the direct link between pressure and solubility.
Another fundamental example is the ability of fish and other aquatic organisms to breathe using gills, which extract dissolved oxygen molecules from the surrounding water. Without the slight solubility of atmospheric oxygen, marine life could not survive in the water column. On a global scale, the oceans act as a massive sink for atmospheric carbon dioxide, absorbing billions of tons of the gas annually. This high solubility, while initially helping to regulate the atmosphere, leads to ocean acidification, altering the chemistry of seawater and impacting marine ecosystems.