Boiling water is a common purification method, but the process changes when the water is already extremely pure, such as distilled water. Distilled water is created specifically to remove impurities, raising questions about how boiling it differs from boiling standard tap water. This article explores the unique physical properties involved in heating ultra-pure water and the safety considerations for its consumption and practical use.
Defining Distilled Water and Its Purity
Distilled water is purified water created through a process mimicking the natural hydrologic cycle of evaporation and condensation. Water is heated until it turns into steam, leaving behind virtually all non-volatile substances. The steam is then cooled, condensing back into liquid water nearly devoid of minerals, dissolved solids, and most contaminants.
This purification method effectively removes inorganic compounds like heavy metals and nitrates, along with microorganisms such as bacteria and viruses. The resulting liquid contains a total dissolved solids (TDS) content often below one part per million, making it exceptionally pure. Tap water, in contrast, contains varying levels of naturally occurring minerals, electrolytes, and sometimes trace contaminants depending on the source and treatment.
The Physics of Boiling Ultra-Pure Water
The physical properties of distilled water mean it can behave unexpectedly when heated, particularly in a clean or smooth container. Boiling is initiated by the formation of vapor bubbles, which typically start at nucleation sites. These sites are small imperfections on the container surface or dissolved solids and air pockets in the liquid. Since distilled water is extremely pure and lacks these dissolved solids and trapped gases, it also lacks the typical nucleation sites needed for controlled bubble formation.
Without these sites, the liquid can be heated past its normal boiling point of 100°C without boiling, a phenomenon called superheating. This superheated state is unstable, and any slight disturbance, such as a bump or the addition of a stirring utensil, can cause the water to suddenly and violently flash into steam. This explosive vaporization is often referred to as “bumping” and can spray scalding hot water, posing a burn hazard. To boil ultra-pure water safely, introducing a boiling chip or a slightly rough surface provides the necessary nucleation sites for normal, gentle bubbling.
Safety Concerns for Consumption and Practical Uses
Safety for Consumption
While boiling distilled water is physically safe, the concern for consumption relates to its lack of mineral content. Distillation removes beneficial minerals like calcium and magnesium, which contribute to flavor and offer minor nutritional support. When consumed, demineralized water is considered “aggressive” because it seeks to achieve equilibrium by absorbing minerals from whatever it contacts, including storage containers or body tissues.
The World Health Organization notes that drinking water low in calcium and magnesium may contribute to a low intake of these elements. However, for most people with a balanced diet, the minerals obtained from water are a small fraction of their total intake. Therefore, drinking distilled water in moderation is generally not a significant health risk, and its purity can be an advantage if local tap water is heavily contaminated.
Practical Uses
Distilled water is highly valued specifically because its lack of dissolved solids makes it ideal for non-consumption applications. The absence of minerals means it will not leave behind scale or deposits when it evaporates or turns to steam. This property is useful for sensitive machinery where mineral buildup can cause damage or reduce efficiency.
Common applications include filling Continuous Positive Airway Pressure (CPAP) machines, where mineral deposits could damage the heating element. It is also the preferred choice for steam irons, car cooling systems, and lead-acid batteries to prevent corrosion and scale formation. In laboratory and medical settings, its high purity is required for sterilizing equipment and preparing chemical solutions where trace elements could interfere with results.