The general public often equates “clean” tap or bottled water with “pure” water. While municipal water is treated to be safe for consumption, it still contains various dissolved substances that give it flavor and electrical conductivity. True pure water, as defined by science, is a highly specific chemical compound that is exceedingly rare in nature. Achieving the highest levels of purification requires sophisticated processes, usually only necessary for industrial, laboratory, or medical applications.
The Scientific Definition of Pure Water
Chemically, pure water is defined as only molecules of H₂O, devoid of all dissolved minerals, gases, organic compounds, and microbial life. This substance possesses a neutral pH of 7.0 and cannot conduct electricity because it lacks the charged ions necessary for current flow. The presence of any dissolved salt or mineral introduces ions and immediately increases conductivity.
Under standard atmospheric pressure, the physical properties of pure water are unvarying; it freezes at 0°C (32°F) and boils at 100°C (212°F). The addition of any solute, such as salt, alters these boiling and freezing points. This is why naturally occurring water, like rainwater or spring water, is never truly pure. Even water vapor contains trace amounts of dissolved gases, such as carbon dioxide, which slightly lower its pH.
Methods of Achieving Water Purity
Creating water that consists only of H₂O requires multi-stage filtration and purification systems.
Distillation
Distillation is one of the oldest and most effective methods, involving heating the water until it vaporizes into steam, leaving non-volatile contaminants behind. The steam is then cooled and condensed back into liquid form, resulting in highly pure water. Distillation is excellent at removing heavy metals and salts, but it may not effectively remove certain volatile organic compounds (VOCs) that can vaporize along with the water.
Reverse Osmosis (RO)
Reverse osmosis (RO) is a physical separation technique where high pressure forces water molecules through a semi-permeable membrane. This membrane blocks virtually all larger molecules and dissolved ions, including salts and many microorganisms, resulting in a rejection rate of up to 99% of dissolved solids. RO is often used as a pretreatment step before more demanding purification processes.
Deionization (DI)
Deionization (DI) is a targeted chemical process that uses ion exchange resins to remove charged mineral salts. The water passes through resin beads that swap positively charged ions (cations) for hydrogen ions, and negatively charged ions (anions) for hydroxide ions. Since hydrogen and hydroxide combine to form water (H₂O), this process effectively removes ionic impurities. To achieve ultra-pure water, such as reagent-grade water used in sensitive laboratory experiments, these three methods are frequently used in sequence, often with additional carbon filtration or ultraviolet sterilization.
Common Contaminants Removed
Achieving purity requires eliminating various classes of unwanted substances naturally present in water sources.
Dissolved Solids and Minerals
This major group includes hardness minerals like calcium and magnesium, as well as dissolved salts. Purification methods specifically target the removal of these ions, which contribute to the water’s measurable total dissolved solids (TDS) content.
Organic and Chemical Contaminants
These contaminants range from industrial solvents and pesticides to chlorine byproducts. Activated carbon filters are effective at adsorbing many of these organic compounds, especially those that contribute to unpleasant tastes and odors. Removing these traces is important for safety and for applications where chemical interference must be avoided.
Biological Agents
Biological agents, such as bacteria, viruses, and protozoa, must also be eliminated. While filtration membranes in RO systems can physically block many pathogens, disinfection steps like boiling or exposure to ultraviolet (UV) light are often incorporated.
Drinking Pure Water: Health and Safety Considerations
A frequent question is whether pure water is suitable for regular consumption. Since pure water lacks all dissolved minerals, it also lacks beneficial electrolytes like calcium and magnesium found in tap and mineral water. These trace minerals are a small but beneficial part of a person’s daily intake.
From a physiological perspective, pure water is hypotonic, meaning it has a lower concentration of solutes than the fluids and cells in the human body. When consumed, this hypotonicity can cause water to be absorbed more rapidly into the bloodstream as the body attempts to balance the concentration gradient. While occasional consumption is harmless, the exclusive, long-term drinking of pure water could lead to minor electrolyte imbalances, as the body struggles to maintain internal osmotic pressure.
The flat taste of pure water is a direct consequence of removing all dissolved gases and minerals, which normally contribute to the flavor profile. The water produced by purification methods is primarily designed for non-potable uses, such as in laboratories, electronics manufacturing, or medical sterilization, where the absence of impurities is paramount. For human consumption, water that contains a balanced level of minerals and is within safety standards, known as potable water, is the most appropriate choice.