Distillation mimics the natural water cycle of evaporation and condensation. This technique is highly effective because it leverages the fundamental difference in physical properties between water and the contaminants it may contain. Understanding exactly what is removed, and what is not, clarifies why distilled water is valued for its exceptional purity.
The Mechanism of Distillation
Distillation purifies water by relying on different boiling points. The source water is heated until it reaches its boiling point of 212°F (100°C). As the water turns into steam, the vast majority of dissolved solids and non-volatile impurities are left behind because their boiling points are significantly higher than water’s.
The steam then rises and enters a separate cooling area, often containing condensing coils. Here, the steam loses thermal energy and cools rapidly, converting the water vapor back into liquid water droplets. This highly purified liquid is collected in a separate storage container, completing the process.
Non-Volatile Substances Eliminated
The distillation process is effective at removing non-volatile substances. Inorganic minerals and salts are a major category of removed contaminants, including hardness-causing agents like calcium and magnesium, as well as sodium, chloride, and sulfate ions. These dissolved solids, which can cause scale buildup and alter the taste of water, are left behind as a concentrated residue.
Distillation also eliminates heavy metals, which are a health concern. Toxic elements are successfully separated because they remain fixed in the boiling chamber during evaporation. Examples include:
- Lead
- Mercury
- Arsenic
- Barium
This separation is reliable, capable of removing up to 99.5 percent of total dissolved solids.
Biological contaminants are fully addressed by the high temperatures reached during the purification cycle. Pathogens like bacteria, viruses, and protozoa are inactivated and killed by the sustained boiling process. Their remnants are left behind with the other non-volatile solids.
Contaminants That Can Remain
Distillation is less effective against substances that have a boiling point similar to, or lower than, water. Volatile Organic Compounds (VOCs) are the primary example of contaminants that can pass through the system. Substances like benzene, toluene, and certain pesticides or solvents can vaporize along with the water steam.
These compounds can travel with the steam and re-condense into the final product. This includes industrial solvents and petroleum byproducts, which can re-contaminate the water and impart an off-flavor. For this reason, many modern distillation units incorporate a post-purification step, such as an activated carbon filter, to adsorb residual VOCs.
Dissolved gases are another group of substances not fully removed by distillation and may re-enter the water after condensation. Atmospheric gases like oxygen and carbon dioxide, naturally dissolved in tap water, can travel through the vapor path or dissolve back into the water upon collection. The removal of oxygen is why many people describe the taste of distilled water as “flat” or “bland.”
Practical Uses of Distilled Water
The high purity achieved by distillation makes this water indispensable where dissolved solids would be detrimental. In laboratory and medical settings, distilled water is required for preparing chemical solutions, sterilizing equipment, and use in kidney dialysis machines.
In household and automotive contexts, distilled water is valued for its inability to cause mineral buildup or corrosion. Using it in steam irons, humidifiers, and CPAP machines prevents the formation of scale, which can clog internal components and shorten the life of the appliance. Similarly, it is used in car batteries and radiator systems to prevent mineral deposits from corroding metal parts and ensuring optimal function.