Tap water is a complex solution containing various dissolved substances. One of the most common is the invisible gas, carbon dioxide (CO2). This gas is naturally present in all water sources, absorbed from the atmosphere and the surrounding environment. The interaction between CO2 and water molecules initiates a chemical change that impacts the water’s properties and the infrastructure it travels through.
The Immediate Chemical Reaction
When carbon dioxide gas dissolves into water, a portion of the gas immediately reacts with the water molecules. This chemical process is known as hydration, and it results in the formation of a new compound. The reaction is represented by the formula: CO2 + H2O ↔ H2CO3.
Carbonic acid (H2CO3) is a weak acid. The reaction is an equilibrium process, constantly forming and breaking back down into carbon dioxide and water. While the majority of CO2 remains dissolved, the small percentage that forms carbonic acid drives the noticeable changes.
Carbonic acid partially dissociates in the water solution. This dissociation releases hydrogen ions (H+) and bicarbonate ions (HCO3-). The presence of these hydrogen ions increases the water’s acidity.
Consequences for Plumbing and Water Systems
The formation of carbonic acid in tap water leads directly to a reduction in the water’s pH level, a process known as acidification. Water quality standards typically aim for a neutral or slightly alkaline pH to ensure the water remains stable. When the pH drops below 7.0, the water becomes more corrosive, affecting the metal infrastructure of distribution networks and household plumbing.
This corrosivity causes the water to begin dissolving the materials that make up the pipes. The carbonic acid facilitates the leaching of trace amounts of metals from the interior surface of the plumbing. This is particularly problematic in older homes where materials like copper, iron, and lead are present in the pipes, fixtures, or solder joints.
For example, the acid dissolves copper from pipes, resulting in blue-green staining on sinks and fixtures. It can also dissolve iron from galvanized steel pipes, manifesting as rusty water and a metallic taste. Most significantly, acidic water causes the leaching of lead from solder and brass fixtures, posing a serious health risk.
The long-term impact on infrastructure involves the gradual weakening and degradation of the entire water system. This corrosion can lead to pinhole leaks in copper piping and a reduction in the service life of appliances that use water. Consequently, water treatment facilities often employ methods to adjust the water’s pH, typically by adding alkaline substances, to slow this corrosive effect before the water enters the distribution system.
Sources and Sensory Experience
The carbon dioxide found in tap water originates from both natural environmental processes and human-controlled sources. In nature, groundwater acquires CO2 as rainwater absorbs it from the atmosphere and then as the water percolates through soil and rock containing decaying organic matter. This natural absorption can lead to high concentrations of dissolved gas in water drawn from deep wells.
Artificial sources contribute to the CO2 levels through water treatment processes and intentional carbonation. Sometimes, carbon dioxide is intentionally added to water during treatment to aid in the softening process or to adjust the pH for stability. In the home, the most common artificial source is the use of carbonation devices, which forcibly inject CO2 under pressure to create sparkling water.
From a consumer perspective, the presence of dissolved carbon dioxide provides a distinct sensory experience. The chemical reaction that forms carbonic acid gives the water a slightly acidic, or “tart,” flavor. This perception is what people associate with the “crispness” or “sharpness” of carbonated beverages.
When CO2 concentrations are very high, the gas may come out of solution as tiny bubbles, creating the effervescence of soda water. Even in typical tap water, the subtle tangy flavor is a direct result of the weak carbonic acid created by the dissolved gas. This natural taste profile is generally considered pleasant unless the CO2 is contaminated with other compounds that impart off-flavors.