The abbreviation dH₂O stands for distilled water, and you will encounter it on labels in laboratories, pharmacies, and equipment manuals alongside a confusingly similar term: deionized water (often written as DI water). Both are forms of purified water, but they reach that purity through fundamentally different processes, they remove different types of contaminants, and they are not interchangeable for every purpose. Understanding which is which matters if you work in a lab, maintain equipment that calls for purified water, or simply wonder whether drinking either one is a good idea.
How Distillation Produces dH₂O
Distillation is one of the oldest purification methods humans have used. The principle is straightforward: you heat water until it evaporates, then cool the resulting steam so it condenses back into liquid form. Because water boils at 100 °C and most dissolved minerals, salts, and heavy metals do not, those contaminants stay behind in the boiling vessel. The condensed liquid that collects on the other side is distilled water.
The process removes more than just dissolved ions. Bacteria, viruses, and most organic compounds also get left behind because they cannot travel with the steam. This makes distilled water broadly clean in a way that goes beyond just removing mineral content. However, distillation is not perfect. Certain volatile organic compounds that have boiling points close to or below that of water can carry over into the distillate. That is why advanced distillation systems sometimes include additional filtration stages or use multiple rounds of distillation.
Distillation is energy-intensive. Bringing water to a full boil and sustaining it takes significant heat input, and the process is relatively slow compared to other purification methods. A benchtop laboratory still might produce only a few liters per hour. This energy cost is one reason why distillation has gradually ceded ground to other technologies in settings where very large volumes of purified water are needed.
How Deionization Works
Deionization takes a completely different approach. Instead of phase-changing the water, it passes liquid water through specialized resin beds that chemically swap out dissolved ions. A cation exchange resin grabs positively charged ions like calcium, magnesium, and sodium and replaces them with hydrogen ions. An anion exchange resin grabs negatively charged ions like chloride, sulfate, and bicarbonate and replaces them with hydroxide ions. The released hydrogen and hydroxide combine to form more water molecules, leaving you with liquid that has had its ionic content stripped away.
Research on ion exchange resins has demonstrated that cation resins can reduce salinity effectively while anion resins bring down levels of sulfate, chloride, bromide, and fluoride to acceptably low concentrations.1Minerals Engineering. Treatment of acid mine water by use of heavy metal precipitation and ion exchange The process is fast and can be scaled up easily, which is why many laboratories and industrial facilities prefer it for day-to-day operations.
The critical limitation of deionization is that it only removes charged particles. Bacteria, pyrogens (fever-causing fragments from bacterial cell walls), organic molecules that carry no electrical charge, and dissolved gases all pass through the resin beds untouched. A bottle of freshly made DI water can be free of minerals yet still harbor microorganisms. That distinction is the single most important practical difference between distilled and deionized water.
What Each Method Removes and What It Misses
The easiest way to think about the difference is by category of contaminant:
- Dissolved minerals and salts: Both distillation and deionization remove these effectively. Calcium, magnesium, sodium, chloride, sulfate, and similar ions are stripped out by either process.
- Bacteria and viruses: Distillation kills and removes these because nothing biological survives the boiling-and-condensing cycle. Deionization does not address them at all.
- Organic compounds: Distillation removes most organics, except volatile ones with low boiling points. Deionization removes only those organics that carry an ionic charge, which is a small subset.
- Dissolved gases: Neither process reliably removes dissolved gases like carbon dioxide or oxygen. CO₂ in particular re-dissolves quickly from the air and forms carbonic acid, which is why even freshly purified water tends to be slightly acidic rather than perfectly neutral.
- Particulates: Distillation removes particulates because they do not evaporate. Deionization does not inherently filter particles, though many DI systems include a pre-filter to protect the resin beds.
These differences mean the two types of water are not always substitutable. A chemistry experiment that needs water free of ionic interference might be perfectly served by DI water. A microbiology lab preparing sterile media needs water free of biological contamination, making distilled (or autoclaved DI) water the safer choice.
Measuring How Pure the Water Actually Is
Purity in water is commonly gauged by its electrical resistivity. Pure water conducts electricity poorly because there are very few ions available to carry charge. Ordinary tap water has a resistivity somewhere around a few thousand ohm-centimeters, depending on your local mineral content. Freshly produced distilled or deionized water reaches a resistivity closer to a few megohm-centimeters. The theoretical maximum for perfectly pure water at 25 °C is 18.2 megohm-cm, a benchmark that only ultrapure water systems approach.
Resistivity meters and their inverse, conductivity meters, are standard equipment in any lab that cares about water quality. A quick conductivity reading tells you whether your DI resin is exhausted or whether your still is producing clean distillate. When the reading starts climbing, the resins need regenerating or replacing, or the still needs maintenance. This measurement tells you about ionic purity specifically, though. It will not warn you about bacteria or uncharged organics, which is why labs that need the highest purity water also run total organic carbon tests and microbial counts.
Where Each Type Gets Used
Distilled water appears wherever biological purity matters or where an established protocol specifies it. Autoclaves, cell culture media, pharmaceutical formulations, and CPAP machines all commonly call for distilled water. It is also the traditional choice for topping off lead-acid batteries, because the minerals in tap water would slowly poison the battery’s electrodes. When you see dH₂O on a laboratory protocol sheet, the author chose distilled water for a reason, and substituting DI water without checking whether microbial contamination matters could cause problems.
Deionized water dominates in applications where ionic purity is the priority and biological contamination is either irrelevant or handled separately. Analytical chemistry, rinsing glassware, preparing reagent solutions, and cooling systems that are sensitive to mineral scale all rely on DI water. The semiconductor industry, in particular, consumes staggering quantities of ultrapure water for wafer cleaning, photolithography, and chemical vapor deposition, processes where even parts-per-trillion levels of ionic contamination can ruin a chip.2VNU Journal of Science: Earth and Environmental Sciences. Technologies for Ultrapure Water Production in Semiconductor Industry
In semiconductor fabrication, neither distillation nor simple deionization alone is sufficient. Facilities use multi-stage systems combining reverse osmosis, deionization, ultraviolet oxidation, and ultrafiltration to push water purity to levels far beyond what a laboratory still or a benchtop DI cartridge can achieve.3Desalination. Recent transitions in ultrapure water (UPW) technology: Rising role of reverse osmosis (RO) This “ultrapure” water is a category unto itself, and its production is one of the more resource-intensive behind-the-scenes aspects of electronics manufacturing.
Is Distilled or Deionized Water Safe to Drink?
Drinking a glass of distilled water will not harm you. Your intestines absorb distilled water normally. Research measuring fluid movement in the human small intestine found that distilled water was absorbed at a rate comparable to an electrolyte-carbohydrate solution, and faster than a concentrated glucose drink that actually caused net fluid secretion into the gut.4PubMed. Human intestinal water absorption: direct vs. indirect measurements Your body does not reject distilled water or struggle to process it in the short term.
The concern with long-term exclusive use of demineralized water is subtler. Drinking water that is completely stripped of calcium, magnesium, and other minerals means missing out on a dietary source of those minerals that most people take for granted. A narrative review of the evidence concluded that the combined effect of consuming low-mineral water alongside ongoing urinary excretion of minerals could contribute to demineralization of bones and teeth, raising the risk of osteoporosis and dental problems over time.5PubMed Central. The Role of Low Mineral Water Consumption in Reducing the Mineral Density of Bones and Teeth: A Narrative Review This does not mean a few weeks of distilled water will weaken your skeleton. It means that if demineralized water is your sole water source for months or years, and your diet is not compensating with adequate mineral intake from food, the gap could eventually matter.
Deionized water that has not been distilled or otherwise sterilized is a different story. Because DI resins do not remove bacteria, drinking DI water straight from a laboratory system is not advisable. Some DI systems can develop bacterial colonies on the resin beads themselves, particularly if the system sits unused for long periods. For drinking purposes, distilled water is the safer of the two if you insist on purified water, though for most people, filtered tap water with its natural mineral content is perfectly fine and arguably better.
The Taste Issue
People routinely describe distilled and deionized water as tasting “flat” or “empty.” This is not imagination. The dissolved minerals in ordinary drinking water contribute to its taste in ways that are easy to overlook until they are absent. Calcium lends a slight sweetness, magnesium can add a bitter edge, bicarbonate gives water a familiar mouthfeel. Strip all of that out and you are left with something that tastes noticeably different from what your palate expects.
Concerns have been raised about whether long-term consumption of such demineralized water is prudent, particularly in regions that rely on desalination for their water supply. Reverse osmosis, the dominant desalination technology, produces water that is similarly depleted of minerals. Some researchers have questioned whether this water is safe for prolonged use without remineralization.6PubMed Central. Demineralization of drinking water: Is it prudent? In practice, most municipal desalination plants add minerals back into the water before distributing it, partly for health reasons and partly because the public simply does not like the way fully demineralized water tastes.
Why Purified Water Is Corrosive
One of the more counterintuitive properties of highly purified water is that it is aggressive toward metals, concrete, and even some plastics. Water that contains dissolved minerals is in a kind of chemical equilibrium; the minerals partially satisfy water’s tendency to dissolve things. Remove those minerals, and the water becomes “hungry” again. It will leach ions from whatever container it sits in, dissolving trace amounts of metal from pipes, pulling calcium from concrete, and gradually degrading seals and fittings.
This is why laboratories store distilled and deionized water in high-density polyethylene (HDPE) or borosilicate glass containers rather than metal ones. It is also why industrial systems that use purified water for cooling or rinsing need to account for material compatibility in their plumbing. Stainless steel holds up reasonably well, but even it can suffer over time under certain conditions, particularly if biofilms develop on the surface and create local chemistry that accelerates corrosion.
For the same reason, you should not use distilled or deionized water in plumbing fixtures or appliances designed for tap water. The water’s corrosive tendencies are minor at household volumes and timescales, but if you are running purified water through copper pipes for months, you may end up with more dissolved copper in your water than you started with, defeating the purpose of purification.
Shelf Life and Contamination After Purification
Neither distilled nor deionized water stays perfectly pure for long once it is exposed to the environment. Carbon dioxide from the air dissolves into the water within minutes, forming carbonic acid and dropping the pH from roughly 7 to somewhere around 5.5 or 6. Dust particles settle in. In the case of DI water, any microorganisms present at the time of production begin multiplying, since there are no disinfectant residuals to keep them in check.
Laboratories that need high-purity water typically produce it on demand from a point-of-use system rather than storing large volumes. A DI polishing cartridge mounted directly at the bench dispenses water that is used immediately, minimizing the window for contamination. When storage is unavoidable, sealed containers made of inert materials, kept in a clean environment, are standard practice. Even so, most labs treat stored purified water with some skepticism and re-test it before using it for sensitive work.
If you buy distilled water at a grocery store, the sealed jug is fine for household uses like irons, humidifiers, and CPAP machines. Once opened, treat it like any other container of water: it will not spoil quickly, but it is no longer sterile, and its purity is gradually declining.
The Environmental Side of Producing Purified Water
Distillation’s environmental footprint is dominated by energy. Boiling water requires heat, and unless that heat comes from a renewable source, you are burning fuel to produce what is essentially very clean water. For small laboratory stills, the energy cost per liter is meaningful but manageable. For large-scale industrial distillation, it becomes a serious economic and environmental consideration, which is one reason why reverse osmosis and deionization have largely replaced distillation for high-volume applications.
Deionization has its own environmental costs, centered on the resin regeneration cycle. Ion exchange resins eventually become saturated with the ions they have captured and need to be regenerated using strong acids and bases, typically hydrochloric acid for cation resins and sodium hydroxide for anion resins. These regenerant chemicals produce a waste stream of concentrated salt and acid that must be neutralized and disposed of properly. Life cycle assessments of resin-based systems, particularly those used for treating contaminated water, have found that the disposal method for spent regeneration solutions significantly affects overall environmental impact, with incineration of waste solutions carrying the heaviest burden.7PubMed Central. Life cycle environmental impacts of regeneration options for anion exchange resin remediation of PFAS impacted water
Modern systems try to minimize waste by recycling components of the regeneration solution or by using mixed-bed cartridges that are returned to the manufacturer for centralized, more efficient regeneration. Electrodeionization, a newer technology that uses electricity to continuously regenerate resin in place, eliminates chemical regeneration entirely for certain applications, though it requires a consistent power supply and works best as a polishing step after reverse osmosis has done the heavy lifting.
When the Label Says “Purified” Instead
If you shop for water at a store, you will notice that some bottles say “distilled,” some say “purified,” and some say “spring” or “mineral.” In the United States, “purified water” is a regulated term that means the water has been treated to meet certain purity standards, but it does not specify the method. Purified water might have been distilled, deionized, run through reverse osmosis, or processed using some combination of these. The label “distilled water” specifically means distillation was the primary process.
For most household purposes, the distinction between “purified” and “distilled” is academic. If your CPAP manual says to use distilled water, distilled is the safest bet because it guarantees both mineral removal and microbial kill. If your steam iron just says “purified,” any of the above options will prevent mineral buildup in the heating chamber. Where the distinction truly matters is in laboratory and clinical settings, where the specific purity profile of the water can affect experimental results or patient safety.
Pharmacies and hospitals typically use water labeled as “Water for Injection” (WFI) for preparing injectable medications. WFI has historically been required to be produced by distillation in many countries, precisely because distillation’s ability to remove pyrogens made it the gold standard for water that would enter the bloodstream. Some regulatory agencies have recently expanded the allowed production methods to include reverse osmosis combined with ultrafiltration, reflecting improvements in membrane technology. But the principle remains the same: when biological purity is critical, the production method matters, and knowing whether your water is distilled, deionized, or something else is not just pedantry.