The molecular formula for water, H₂O, suggests a simple, uniform substance, yet what comes out of a faucet or a bottle is seldom just pure H₂O. Water is often referred to as the universal solvent because it readily dissolves substances it encounters. The differences in drinking water are defined not by the water molecule itself, but by the collection of dissolved solids, gases, and microscopic life mixed within it. These variations are influenced by the water’s source, the geological path it travels, and the treatments it undergoes.
Differences in Dissolved Solids and Minerals
The primary factor distinguishing one water source from another is its concentration of Total Dissolved Solids (TDS), which measures all inorganic and organic substances dissolved in the water. These solids include minerals, salts, and trace metals, and are typically measured in parts per million (ppm). The U.S. Environmental Protection Agency (EPA) recommends that drinking water not exceed 500 ppm of TDS, as higher levels can affect the water’s taste and appearance.
A major component of TDS is water hardness, which tracks the concentration of multivalent cations, primarily calcium and magnesium ions. Water is classified as “hard” when it contains high levels of these minerals, naturally picked up as water flows through rock formations like limestone. These minerals contribute to the distinct taste of hard water, but they also react with soap, reducing its ability to lather and leaving behind mineral deposits, known as scale, on fixtures and appliances.
Water with low mineral content is known as “soft water,” which allows soap to suds easily but can taste flat to some palates. In contrast, water with a high TDS level, particularly exceeding 500 ppm, may have a bitter, salty, or metallic flavor due to concentrated ions like sodium and sulfate. The composition of dissolved solids influences the water’s sensory profile and its utility for household tasks.
Distinctions Based on Source and Regulatory Oversight
Drinking water in the United States is governed by two separate federal agencies, creating a fundamental distinction in oversight. Municipal tap water is regulated by the EPA, which sets national standards for contaminants and requires public water systems to test frequently and disclose results. This regulation ensures the water is treated to remove pathogens and meet safety standards as it travels through the public distribution network.
Bottled water, however, is regulated as a packaged food product by the Food and Drug Administration (FDA). The FDA establishes “Standards of Identity” that define various types of bottled water, such as spring water, mineral water, and purified water. Spring water must be collected from an underground source that flows naturally to the surface and must retain the same composition as it had at its source.
Mineral water is defined by its naturally occurring TDS, requiring at least 250 ppm of dissolved solids, with no minerals allowed to be added. Purified water, on the other hand, can come from any source, including municipal tap water, but must be treated extensively to remove almost all solids and contaminants. Bottlers must clearly state the water’s source and the treatment method used on the label.
The Impact of pH and Alkalinity
The pH of water, or the potential of hydrogen, measures its acidity or alkalinity on a scale from 0 to 14. A pH of 7 is neutral; values below indicate increasing acidity, and values above represent increasing alkalinity. The pH level is naturally determined by the geology of the source, as water passing through granite tends to be more acidic, while water flowing through limestone is more alkaline.
The EPA generally recommends that public drinking water maintain a pH between 6.5 and 8.5. Water with a low pH can become corrosive, potentially leaching metals like copper or lead from plumbing fixtures into the drinking supply. Conversely, highly alkaline water can sometimes impart a bitter taste and contribute to scale formation in pipes.
Alkalinity is closely related to pH but represents the water’s capacity to neutralize acids, known as its buffering capacity. This buffering ability is primarily due to the presence of bicarbonate, carbonate, and hydroxide ions in the water. The current consumer interest in “alkaline water” involves products that have an elevated pH, typically ranging between 8 and 10, often achieved through ionization or the addition of minerals.
The Concept of Highly Purified Water
Highly purified waters represent the extreme end of the treatment spectrum, where nearly all dissolved solids are intentionally removed. Distilled water is produced by boiling the water into steam, which is then condensed back into a liquid state, leaving behind minerals, salts, and other impurities. This process makes the resulting water suitable for applications where mineral buildup is undesirable, such as in humidifiers, car batteries, and medical equipment.
Deionized (DI) water, also known as demineralized water, is created by passing water through electrically charged resin beds in an ion exchange process. This method effectively removes charged mineral ions like calcium and chloride but is not designed to remove uncharged organic molecules, such as viruses or bacteria. DI water is often used in laboratories and industrial processes requiring water with extremely low ion content.
Both distilled and deionized water are chemically very close to pure H₂O, resulting in a flat or neutral taste due to the absence of flavor-contributing minerals. While generally safe for occasional consumption, these highly purified forms are not usually recommended for long-term daily drinking because they lack the beneficial minerals found in tap or spring water. The absence of dissolved ions can make deionized water slightly more reactive and mildly corrosive.