The process of treating water to remove contaminants is common, but the terminology used to describe the final product can often be confusing. Terms like “distilled water” and “purified water” are frequently used interchangeably, yet they represent products created through fundamentally different processes and possessing distinct characteristics. Both types of water are cleaner than standard tap water, but the methods employed and the resulting level of purity separate them into different categories. Understanding these differences in production and final composition is necessary for selecting the appropriate water for a given use.
Defining Purified Water
“Purified water” is a broad classification for water that has been mechanically treated to remove chemicals and contaminants. This water must meet a specific standard for cleanliness, typically defined by a governing body such as the United States Pharmacopeia (USP). The defining characteristic is the reduction of impurities to 10 parts per million (PPM) of Total Dissolved Solids (TDS) or less.
Achieving this standard involves a variety of techniques, often used in combination. Common methods include reverse osmosis (RO), which forces water through a semi-permeable membrane to trap dissolved solids. Deionization (DI) is another technique that uses specialized resins to remove mineral ions from the water.
Activated carbon filtration is often used as a pretreatment step to remove chlorine and organic compounds that can affect taste and odor. The goal is a product significantly cleaner than tap water, regardless of the purification method used to achieve the target TDS limit.
Understanding Distilled Water
Distilled water represents a specific method of water treatment that relies on a natural physical process. Production involves boiling the source water until it turns into steam, leaving behind virtually all non-volatile substances. This steam is then cooled in a separate chamber, where it condenses back into liquid water.
This method is exceptionally effective at removing contaminants that do not vaporize easily, such as inorganic minerals, salts, and heavy metals. The high heat of the boiling process also kills most bacteria and other pathogens. Because the water is separated from the contaminants through a phase change, distillation often achieves a higher purity level than many single-stage purification techniques.
The process physically separates the pure water molecules from the impurities, which remain as a concentrated residue in the boiling chamber. This technique contrasts with methods like reverse osmosis, where contaminants are filtered out under pressure. Distilled water is a subset of purified water, but it is created through a distinct physical separation process.
Purity Levels and Trace Elements
The primary difference between the two types of water is their final purity measurements, specifically the level of Total Dissolved Solids (TDS). Purified water must have a TDS measurement of 10 PPM or less to meet its classification standard. Water produced by an RO system typically measures between 1 and 10 PPM, retaining trace dissolved elements.
In contrast, distillation results in water with an extremely low TDS measurement, often registering at 0.5 PPM or virtually zero. Distillation removes nearly all trace elements, including beneficial minerals like calcium and magnesium, with over 99.9% efficiency. This near-total removal of minerals is why distilled water is chemically neutral and has a characteristic flat or bland taste.
Purified water, depending on the method and whether remineralization is performed, can retain a small amount of these trace elements. This difference is the determining factor in the water’s taste and its potential for leaving behind mineral deposits. The presence of dissolved solids in purified water distinguishes it from the near-perfect demineralization of distilled water.
Practical Applications and Selection
The choice between purified and distilled water is dictated by the intended use, based on the presence or absence of trace elements. Distilled water is preferred for any application where mineral deposits could cause damage or interfere with a process. The total lack of dissolved solids prevents mineral buildup, known as scale, inside sensitive machinery.
Distilled water is mandatory for use in devices such as Continuous Positive Airway Pressure (CPAP) machines, steam irons, and laboratory autoclaves. In a lab setting, its high purity ensures that trace elements do not skew the results of sensitive chemical analyses. It is also used in automotive applications, such as topping off lead-acid batteries, to prevent mineral contamination on the plates.
Purified water is the more common choice for general drinking and cooking. Although highly treated, the trace elements or re-added minerals often give it a more palatable taste compared to the flavorless nature of distilled water. For daily consumption, purified water is widely available and is produced more efficiently and cost-effectively than distillation, making it the practical selection for hydration.