Deionised water is water that is almost completely free of dissolved minerals. These minerals exist as cations (e.g., sodium, calcium, copper, iron, magnesium, lead) and anions (e.g., chloride, fluoride, sulphate, nitrate).
Deionised water has a low electrical conductivity and does not form scale or other deposits on surfaces. On its own, deionization does not remove organic compounds, most pathogens, dissolved gases, or other uncharged contaminants. But, following other purification processes, the resulting deionised water is very pure.