Tap water may not serve quite well for washing purposes. Tap water, which contains high amount of minerals, is usually used to clean windows. Pressure washing services also rely on tap water. However, ordinary tap water has dissolved substances. A few of these are calcium, magnesium, and other metal ions. Other nonmetal ions like chloride may also be present. These ions have unwanted effect on the surface of the glass. They react with minerals in the glass and produce undesirable stains. As tap water dries out, what is left on the glass surface is a residue of these particles which looks disagreeable. This is why glass wares become covered with stains, not upon prolonged use but upon frequent washing in tap water.
Deionization is a process that removes water impurities particularly ions. Water is a natural solvent that is why it is not surprising to find it quite impure. Removal of ions in water also removes salts, since salts dissolves in water to give cations (positive ions) and anions (negative ions). For instance when table salt (sodium chloride) dissolves in water, it yields sodium ions (Na+) and chloride ions (Cl-). This means that water does not have molecules of NaCl in the water but ions of Na+ and Cl- dispersed throughout. The same thing is true to all ionic salts. There are several ions commonly found in tap water. Calcium (Ca++), magnesium (Mg++), potassium (K+), iron (Fe+++), and manganese (Mn++) are the cations present in tap water aside from sodium. Sulfates, nitrates, carbonates, and silicates are a few anions aside from chloride. Note that water itself dissociates into H+ and OH- ions.
Deionization occurs in two phases which require resin beds, which are sites for ion exchange to occur. Positive ions are displaced by hydrogen ions in the first phase. The second phase causes negative ions to be displaced by hydroxyl ions. The result is water that contains H+ and OH- ions that combine to form water.
Deionized (DI) water is aggressive. It compensates the loss of minerals or ions by taking them away from the surrounding. This means the DI water is more efficient in removing ions or dirt minerals from surfaces than tap water. Tap water leaves mineral residues on surfaces upon long use. DI water does not because in the first place it has nothing to leave. This means that this kind of water is a better cleaning agent than the other one.
The quality of water that underwent deionization is checked through measuring its resistivity, the ability to resist the flow of electric current. Ions allow water to become an electric conductor. But the less ions found in water the purer it becomes; the less ions the lower the conductivity of water and the higher its resistivity. Following from these statements, the purer the water the higher its resistivity and the more it does not allow electric current to pass.
Resistivity expressed in Meg Ohms is a precise way of measuring water purity in case of deionized or demineralised water. Extremely pure water can have resistivity of 18 Meg Ohms. But less pure variants can be ideal cleaning agents. They are too pure that according to some health experts, if a person drinks too much demineralised water, his ions would leach out of the tissues and this could be potentially dangerous. Nevertheless no enough scientific evidence proves this claim. In fact, another theory says that the absence of minerals in DI water has insignificant effects on humans, which means that demineralised water is no better or worse than mineral water.
When it comes to cleaning, however, the advantage of deionized water over ordinary tap water is quite obvious. The former is in itself an aggressive cleaner that leaves no residue, spots or stains on surfaces. Moreover, it is an excellent agent for rinsing.