Deionized water is also spelled deionised water or called DI water. Another name for it which sounds a little more understandable for many people is demineralised water. However it is called or spelled, it means water that has extremely little ions or minerals in it. Ions are charged atoms. Atoms become charged after gaining or losing at least one electron. A sodium atom (Na) becomes a sodium ion after losing an electron (Na+). A chlorine atom (Cl) becomes a chloride ion (Cl-) after gaining an electron. Metallic salts are composed of ions and not molecules. That is why they are called ionic compounds. The popular example has just been given. Table salt is sodium chloride (NaCl) and it is a popular household ionic substance. For those who have forgotten basic chemistry, NaCl is not composed of molecules of NaCl but is actually composed of ions of Na+ and Cl- bound tightly together by strong electrostatic forces. However, water does the trick in separating these ions. As table salt dissolves in water it dissociates to its component ions. The same thing happens to any other salts in water, and because water is a remarkable solvent, it is never found in pure form, but has always impurities. Filtration and chlorination of water may remove organic impurities and bacteria, but minerals may still be present. These minerals are present in form of ions like calcium (Ca++) and magnesium (Mg++) as well as chlorides, nitrates and carbonates. Though water that contains minerals or ions may not be a health concern, it has some industrial drawbacks. For instance, tap water, which has lots of ion impurities leaves stains or spots on surfaces when used as a cleaning agent. This is where deionization steps in.

Deionization is the process of removing ionic impurities in water. It is also called demineralization. In the industrial scene, this may involve two phases. The first phase removes positive ions of sodium, potassium, calcium, magnesium and iron. They are displaced by hydrogen ions (H+). The second phase removes negative ions like chloride, nitrate, and bicarbonate. These are then displaced by hydroxyl ions (OH-). The resulting water teems with hydrogen and hydroxyl ions, which actually fuse to form water molecule. Both phases use resin beads which serve as an ion exchange site.

The resulting water is said to have no pH value since there are no ions to measure the pH by. However, water that is stripped of its ions is a more aggressive solvent. If left in an open container, it sucks carbon dioxide from the air. This results to an acidic solution causing water to assume a lower pH value. Nevertheless, heating the solution to the boiling point may remove carbon dioxide and restore water’s deionized quality.

There are controversies as to the effects of demineralised water upon drinking it. There is a fear that because it is too pure it may actually be harmful to humans. Extremely pure water will rob the body off its useful electrolytes or ions. The matter with this claim is that it is based upon little evidence.

Industrial purposes of deionized water can never be refuted. It claims extensive application in the semiconductor industry as it is used during processing and cleaning of materials like silicon wafers. The optics industry also relies on this type of highly pure water, since optical surfaces are supposed to be extremely clean as a requirement for coating. Laboratory glasswares are rinsed in DI water as tap water is never recommended for this purpose. Water that is devoid of ions is also used in car wash shops. It is also very suitable and is in fact used in window cleaning. The efficacy of this pure water as a cleaning agent is due to its aggressiveness as a solvent, since water that contains no dissolved ions will tend to draw ions or solutes from the surroundings and surfaces. This means no spots or stains is left on surfaces.

Furthermore, in the manufacture of pharmaceutical and cosmetic products, DI water is often used because it does not contain impurities that may cause unwanted reactions with other substances used in these products.