Deionized water is water that has had its dissolved mineral ions stripped out by ion exchange resin, leaving almost nothing conductive behind. It is not purified in the everyday sense: deionization removes charged particles, so it does nothing about bacteria, and very little about many organic compounds that carry no charge.
How deionization works
Two resins do the job. A cation resin holds hydrogen ions and trades them for positively charged ions in the water, including the calcium and magnesium that make water hard. An anion resin holds hydroxide ions and trades them for negatively charged ions such as chloride, sulfate and nitrate. The released hydrogen and hydroxide combine into more water. Run in series, or blended together in a mixed bed, the pair can take conductivity down to near nothing. When the resin runs out of exchangeable ions it must be regenerated with strong acid and caustic, which is why home DI is usually sold as a throwaway cartridge instead.
Deionized, distilled and reverse osmosis are not the same
Distilled water is boiled and the vapor condensed, so it leaves behind minerals, most microbes and anything that will not evaporate. Reverse osmosis pushes water through a membrane that rejects the great majority of dissolved solids along with larger organisms. Deionization only exchanges ions, and it does it fast and cheaply once the water is already fairly clean. In practice the three are often combined: an RO unit does the bulk removal and a small DI cartridge polishes what is left, which is far more economical than running DI resin on raw tap water.
What people actually use it for
Deionized water goes where dissolved minerals would leave a deposit or interfere with a process: rinsing a car so it dries without spots, topping up lead-acid batteries, filling steam irons and humidifiers, lab glassware, and cooling loops. The value is the absence of scale-forming ions rather than any health property. The USGS defines hardness as dissolved calcium and magnesium, with soft water running from 0 to 60 mg/L as calcium carbonate, and those are precisely the ions a DI bed removes first.
Should you drink it, or plumb it into a house?
Deionization is not a household drinking-water treatment and should not be sold to you as one. It carries no disinfection, so bacteria pass straight through a DI cartridge, and it strips beneficial-tasting minerals along with everything else, which is why DI water tastes flat and slightly sour. If your goal is better-tasting, safer drinking water, a point-of-use reverse osmosis system, optionally with a remineralization stage, is the standard answer. If your goal is spot-free rinsing or scale-free equipment, DI is exactly right for that narrow job.
Questions people ask about what is deionized water
Is deionized water the same as distilled water?
No. Distillation boils the water and condenses the vapor, removing microbes and non-volatile matter. Deionization only exchanges dissolved ions on resin and leaves bacteria and many organics untouched.
Does deionized water remove bacteria?
No, and this is the most common misunderstanding. Resin beds can actually support bacterial growth over time, so DI water is not safe to assume sterile without separate disinfection or filtration.
Can I make deionized water at home?
Yes, with a mixed-bed DI cartridge, and it works best fed by an RO unit so the resin is only polishing. Fed on raw tap water the cartridge exhausts quickly and becomes expensive to keep replacing.
Will deionized water damage plumbing?
Very low mineral water can be mildly aggressive toward some metals over long contact, which is why DI systems are typically plumbed in plastic and used at the point of use rather than distributed through a whole house.