While water is chemically defined as the molecule \(\text{H}_2\text{O}\), consumers categorize it based on its source, processing level, and intended use. This categorization significantly changes the water’s chemical profile and taste. These seven distinct types allow us to distinguish between forms, ranging from municipal tap water to highly refined water used in laboratories. Understanding these types requires looking at how natural processes and human intervention alter the water’s composition, particularly its dissolved solids, mineral content, and \(\text{pH}\) level.
Water Defined by Natural Source
Municipal tap water is the first type, supplied to homes through a public system, typically sourced from rivers, reservoirs, or groundwater. This water is regulated by government agencies, which set limits for over 90 contaminants to ensure safety. Before distribution, it undergoes a multi-step treatment process that includes coagulation, sedimentation, filtration, and disinfection, commonly using chlorine or chloramine.
Spring water, the second type, is derived from an underground formation that flows naturally to the Earth’s surface. Regulations require collection only at the spring or from a borehole tapping the same underground source. As it travels underground, this water naturally filters through rock and soil layers, picking up various minerals that contribute to its distinct taste.
Mineral water, the third type, is closely related to spring water but has a stricter definition based on its composition. It must originate from a geologically protected underground source and contain at least 250 parts per million (\(\text{ppm}\)) of total dissolved solids (\(\text{TDS}\)). No minerals can be added; its mineral profile, which often includes calcium, magnesium, and potassium, must be stable and naturally occurring.
Water Defined by Purification Methods
The fourth type, distilled water, is created by boiling the water to create steam and then cooling the steam to condense it back into liquid. This process leaves virtually all dissolved minerals, salts, and non-volatile impurities behind. The resulting water is exceptionally pure in terms of mineral content, often removing over 99.9\% of dissolved solids.
Distilled water has a flat taste due to the complete lack of minerals and is not the preferred choice for regular drinking. Its primary applications are industrial and medical, such as in lead-acid batteries, automotive cooling systems, and continuous positive airway pressure (\(\text{CPAP}\)) machines, where mineral buildup is undesirable. This high level of purity prevents interference with chemical reactions or the formation of scale on equipment.
Purified water, the fifth type, is a broad category treated to meet stringent standards, often defined as containing no more than 10 \(\text{ppm}\) of \(\text{TDS}\). A common method for creating purified water is Reverse Osmosis (\(\text{RO}\)), which forces water through a semi-permeable membrane under pressure. This membrane has pores small enough to reject up to 99\% of dissolved salts, particles, and many chemical contaminants.
The \(\text{RO}\) process is highly effective at removing impurities like fluoride, chlorine, and heavy metals, making it a popular choice for home filtration systems and the production of most bottled water labeled simply as “purified.” Unlike distillation, \(\text{RO}\) is a continuous process that does not require a phase change, making it more energy-efficient for large-scale production. While \(\text{RO}\) water is clean, it may still contain trace amounts of uncharged contaminants, setting it apart from the absolute purity required for specialized applications.
Water Defined by Chemical Modification
Alkaline water, the sixth type, is chemically modified to have a higher \(\text{pH}\) level than standard tap water, typically ranging from \(\text{pH}\) 8 to 9.5. Water is considered alkaline if its \(\text{pH}\) is greater than 7, which is neutral. This modification is achieved either by adding alkaline minerals, such as calcium and magnesium, or through electrolysis, which separates the water into acidic and alkaline streams.
Proponents suggest that the higher \(\text{pH}\) helps neutralize acid in the body, but scientific support for broad health claims is limited, as the body tightly regulates its internal \(\text{pH}\) balance. Some studies have suggested it may help deactivate pepsin, an enzyme involved in acid reflux, and possibly improve hydration status by lowering blood viscosity. Nevertheless, alkaline water’s primary distinction is its intentional chemical alteration to achieve a specific \(\text{pH}\) for perceived wellness benefits.
The seventh type is deionized water (\(\text{DI}\) water), defined by the removal of virtually all electrically charged ions, or dissolved mineral salts, using ion exchange resins. These resins attract and swap out positive cations and negative anions for hydrogen (\(\text{H}^+\)) and hydroxyl (\(\text{OH}^-\)) ions, respectively, which then combine to form pure \(\text{H}_2\text{O}\). This process is often used as a final step after \(\text{RO}\) filtration to achieve extremely high purity.
Deionized water is not intended for drinking because the deionization process does not remove uncharged contaminants like bacteria or organic compounds, meaning that it is chemically pure but not necessarily microbiologically safe. Its purpose is highly specialized, primarily in laboratory, manufacturing, and industrial settings where the presence of ions would interfere with sensitive processes, such as in electronics manufacturing or specific chemical analyses.