Water terminology often causes confusion, especially when comparing labels like filtered, spring, purified, and distilled. While all represent water that has undergone some level of processing, the specific methods and resulting chemical compositions are quite different. Understanding the processes and the resulting total dissolved solids (TDS) content is key to comparing purified and distilled water. This article defines both types, detailing their production and chemical profiles.
Understanding Distilled Water
Distilled water results from a physical process mimicking the natural water cycle of evaporation and condensation. Production begins by boiling the source water, transforming the liquid into steam. Impurities, such as minerals, salts, heavy metals, and bacteria, are left behind because they do not vaporize at the same temperature as water.
The pure steam is then collected and cooled, returning it to its liquid state. This phase-change process is exceptionally effective at removing total dissolved solids (TDS), typically resulting in a concentration of less than 1 part per million (ppm). Since almost all dissolved substances are removed, the resulting water is demineralized and has a uniquely flat or flavorless profile due to the absence of minerals.
The removal of nearly all ionic compounds means fresh distilled water has extremely low electrical conductivity. A slight decrease in pH, often to around 5.8, occurs when the water absorbs carbon dioxide from the air, forming carbonic acid. This high level of purity makes distillation a standard for applications where foreign substances could interfere with a process or damage equipment.
Understanding Purified Water
Purified water is defined by a quality standard, not a single method, requiring a very low level of total dissolved solids (TDS). To earn the “purified” label, water must have a TDS concentration that does not exceed 10 ppm, a significant reduction from the typical 300 to 400 ppm found in standard tap water. This high level of cleanliness is usually accomplished through multi-stage treatment processes.
Common purification methods include reverse osmosis (RO), which uses pressure to force water through a semipermeable membrane that blocks larger molecules like dissolved salts and contaminants. Deionization (DI) is another technique that uses ion-exchange resins to remove charged mineral ions. Carbon filtration is often used alongside these methods to remove organic compounds, chlorine, and substances that affect taste and odor.
The term “purified” signifies that the water has been aggressively treated to remove impurities, regardless of the original source. Since the definition relies on the final TDS result, it encompasses water treated by any combination of technologies that meets the stringent 10 ppm threshold. This focus on output quality makes purified water a broad category suitable for various uses.
Direct Comparison and Practical Applications
Distilled water is a specific, high-end type of purified water. Distillation is a singular method that reliably produces water with a TDS near 0 ppm, easily meeting the 10 ppm standard for purification. Therefore, all distilled water is purified water, but not all purified water is distilled.
The primary difference lies in the consistency of mineral removal and the methods used. Non-distilled purified water, often produced by reverse osmosis, can retain trace amounts of minerals, which may contribute to a better taste for drinking. Distilled water is essentially mineral-free, which is why it is prone to tasting flat.
The practical application dictates the choice between the two. Distilled water is preferred for technical uses where mineral residue is detrimental, such as in CPAP machines, laboratory work, car batteries, and steam irons. The complete lack of minerals prevents scale buildup that could damage sensitive equipment. Purified water, often produced by RO, is the common standard for most commercially bottled drinking water, offering high cleanliness and a more palatable taste profile.