Boiling water is a common and effective method for microbiological purification, but it does not remove metals or other dissolved inorganic contaminants. For safety concerns related to heavy metals like lead or arsenic, the common practice of boiling water is not only ineffective but can worsen the problem by increasing their concentration. Consumers concerned about water quality must understand the distinction between biological and chemical contamination to select the appropriate treatment method.
Why Boiling Concentrates Metals and Minerals
Boiling water initiates a phase change, converting liquid water (H₂O) into steam (gas), which is the process of evaporation. This physical transformation cleanly separates the water molecules from substances that cannot easily turn into gas at the same temperature. Metals like copper, lead, and arsenic, along with minerals such as calcium and magnesium, are classified as non-volatile compounds.
These inorganic substances exist in water as dissolved solids, meaning they are held in solution and do not evaporate with the water. The boiling points of these heavy metals are extremely high; for example, the boiling point of lead is 1,749°C, which is vastly higher than water’s 100°C. As the water volume decreases during the boiling process, the original mass of dissolved solids remains in the smaller amount of liquid left in the pot.
This creates a concentration effect, similar to reducing a sauce in cooking, where the ratio of contaminant mass to water volume increases. If the water supply contained a concerning level of a heavy metal like lead, boiling off a portion of the water could increase the metal’s concentration by as much as ten percent. Therefore, relying on boiling for chemical purification can result in consuming a higher dose of harmful contaminants per glass.
The total amount of dissolved solids (TDS) in the water, which includes both benign minerals and harmful metals, is not reduced by boiling. Instead, boiling a contaminated water source makes the remaining water chemically less safe, even though it may be microbiologically pure. This is why boiling is never recommended as a solution for known metal contamination.
What Boiling Successfully Removes
While boiling is unsuccessful at removing dissolved inorganic solids, it is a highly effective method for eliminating biological contaminants. The heat generated during a rolling boil is lethal to nearly all waterborne pathogens, including bacteria, viruses, and protozoa. A sustained rolling boil for one to three minutes is sufficient to disinfect water from these microorganisms, making it safe from waterborne diseases.
Boiling can also affect certain organic compounds and gases due to their lower boiling points. Volatile organic compounds (VOCs), which can include chemicals like chloroform and trihalomethanes, may escape into the air as the water is heated. Similarly, chlorine, a common disinfectant, will volatilize and exit the water as a gas, which can improve the taste.
However, the removal of VOCs by boiling is not always advisable because these chemicals can be released into the surrounding air, creating an inhalation risk. The heat from boiling also affects temporary water hardness, which is caused by dissolved calcium and magnesium bicarbonates. The boiling process causes these minerals to convert into insoluble carbonate solids, often visible as scale or deposits on the bottom of a pot, thereby removing them from the liquid solution.
Proven Technologies for Metal Removal
Distillation is one method that relies on the same principle as boiling, but with a critical difference. In a distillation unit, water is boiled to create steam, but this pure steam is then captured and condensed back into a separate container, leaving the non-volatile metals and minerals behind in the boiling chamber.
Reverse osmosis (RO) is a highly effective, pressure-driven filtration method that uses a semi-permeable membrane with microscopic pores. Water is forced through this membrane, which is fine enough to block dissolved inorganic solids, including heavy metal ions, while allowing the pure water molecules to pass through. RO systems can remove up to 99% of heavy metals and other contaminants, making them one of the most comprehensive purification technologies.
Specialized filtration media, such as ion exchange resins, also provide targeted metal removal. Ion exchange resins are polymer beads that chemically swap harmless ions for the problematic heavy metal ions in the water. Activated carbon filters, particularly when modified or combined with other media, can adsorb certain metals, such as lead, onto their large surface area, though they are more commonly used for organic chemicals and taste issues.