The question of whether distilled water contains chlorine is a common concern. The short answer is that distilled water is not chlorinated because the purification process is designed to separate water from nearly all contaminants. Distilled water is created by boiling water into steam and then cooling that steam back into a liquid state. This phase change effectively leaves behind the original water’s impurities.
The Core Answer: Chlorine and Distillation
The removal of chlorine from water during distillation is a matter of physical chemistry, relying on the concept of volatility. Free chlorine, present in tap water as hypochlorous acid, and compounds like chloramines, are considered volatile substances. This means they have a lower boiling point than water, which boils at 100°C (212°F) at standard pressure.
As the source water begins to heat up in the distiller, the chlorine and chloramines convert into a gaseous state well before the water reaches its boiling point. These volatile gasses are driven off early in the process, separating them from the water molecules that are yet to vaporize. The resulting steam that is collected and condensed is therefore virtually free of these chlorine compounds.
Modern distillation units often incorporate vents to expel these volatile gasses, preventing them from mixing with the purified steam. Many distillers also use a final stage of post-filtration, typically an activated carbon filter, to polish the water. This filter captures any trace amounts of chlorine or other volatile organic compounds that may have carried over, ensuring a removal rate that often exceeds 99%.
Why Chlorine is Added to Municipal Water
Adding chlorine to water is a public health measure. Water treatment facilities add chlorine and chloramines primarily as disinfectants to kill or inactivate harmful microorganisms. These pathogens include bacteria, viruses, and parasites that cause waterborne illnesses such as cholera and typhoid.
Chlorination has been instrumental in reducing the spread of these diseases since its widespread adoption in the early 20th century. Beyond the initial disinfection, the added chlorine maintains a “residual” effect. This means the chemical remains active as the water travels through the network of distribution pipes.
This residual disinfectant protects the water supply from potential re-contamination that could occur through leaks or breaks in the pipes. Because chlorine is an effective and persistent disinfectant, it remains the standard choice for safeguarding public drinking water systems.
Comparing Purification Methods
Distillation’s method of chlorine removal through a phase change is distinct from other common water purification technologies. Reverse Osmosis (RO) systems, for example, use high pressure to force water through a semi-permeable membrane. While the RO membrane is highly effective at removing dissolved inorganic solids, it is less consistent at removing volatile compounds like chlorine and chloramines.
RO systems nearly always rely on an upstream activated carbon filter, which protects the membrane from damage and ensures full removal. Standard carbon filtration works differently, using a process called adsorption where chlorine molecules chemically adhere to the surface area of the carbon media.
Deionization (DI) is another method that removes ionized mineral salts but does not involve the heat necessary to drive off volatile chlorine. Distillation relies on the difference in boiling points between water and chlorine compounds. This phase change mechanism makes it uniquely effective at removing a broader spectrum of impurities, including chlorine, without needing additional chemical treatment.