Pure water prepared by distillation is called distilled water. 1. Water in nature is impure, usually containing various salts such as calcium, magnesium and iron, as well as organic matter, microorganisms, dissolved gases (such as carbon dioxide) and suspended solids. The nonvolatile components can be removed by distillation. Distillation method, combined with the following measures, can obtain high quality distilled water. ① Remove the initial fraction (about 20% of the raw water) because the volatile components are mainly concentrated in the initial fraction. ② Discharge the residual part (about 20% of the raw water) because many nonvolatile components are concentrated in the residual water. ③ Add some substances to facilitate distillation. For example, adding NaOH makes CO2 in water become a non-volatile component, and adding KMnO4 can oxidize organic matter in water. One of the quality standards of distilled water is that the salt content is generally around1~ 5 mg/L. Because the salt content in water decreases and the resistivity of water increases, the purity of water can be measured by measuring the resistivity of water. The resistivity of distilled water should be about 0.1×106 Ω cm (ohm cm). Distilled water can be distilled for the second time to obtain heavy distilled water with higher purity. Distillation needs a lot of energy, and now ion exchange method is widely used in laboratory and industry to obtain pure water. This water is also called deionized water.