What does deionized mean?
"Deionized" describes a liquid — almost always water — from which dissolved electrically charged particles, or ions, have been removed. Water is passed through ion-exchange resins that swap out positively charged minerals such as calcium and sodium and negatively charged ones such as chloride, leaving a very pure product. The term is most common in scientific, medical, laboratory, and industrial settings: deionized water rinses electronic components, feeds steam irons and autoclaves, and serves as a base in cosmetics and pharmaceuticals. It sits near "distilled" and "demineralized" in meaning, though each names a different purification method. A memorable fact: unlike ordinary tap water, which conducts electricity because of its dissolved minerals, truly deionized water is a poor conductor — the ions themselves are what carry current.
adjective (past participle of deionize)Of water or another liquid: having had its dissolved mineral ions (such as calcium, sodium, and chloride) removed, typically by passing it through ion-exchange resins. The resulting highly purified water is electrically conductive-poor and used where mineral-free conditions are essential.
- (of water or other liquids) having had dissolved ions removed via ion-exchange purification
"The laboratory autoclaves must be filled with deionized water to prevent mineral scale from building up on the heating elements."
"Electronics manufacturers rinse circuit boards in deionized water to avoid leaving conductive mineral residues behind."
"Fill the steam iron with deionized water to prevent limescale deposits."
Ultra-pure deionized water is actually a poor electrical conductor — it's the dissolved minerals in ordinary water that conduct electricity.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of deionized
"Deionized" combines the privative prefix "de-" (from Latin, indicating removal) with "ion," a term coined by Michael Faraday in 1834 at the suggestion of William Whewell, based on Greek "ion," meaning "that which goes," from the verb "ienai," "to go." The verb "deionize" arose in early 20th-century scientific English as ion-exchange technology developed, and "deionized" functions both as its past participle and as an adjective describing treated water. It shares the Greek root "ienai" with words like "ionic" and "ionosphere."
Related word forms
How deionized is actually used
"Deionized" is used almost exclusively in technical, scientific, laboratory, industrial, and consumer-appliance contexts. It is primarily encountered attributively before "water." In everyday speech people more often say "purified" or "filtered" water; "deionized" signals a specific, high level of purity. Both British and American usage favor the -ize spelling.
Easily confused with deionized
Distilled water is purified by boiling and condensing steam, whereas deionized water is purified by removing ions chemically through ion-exchange resins; the two methods remove overlapping but not identical impurities.
Demineralized is a broader term for water stripped of minerals, while deionized specifically refers to removal achieved by eliminating dissolved ions, often to a higher purity standard.