Water containing no or less soluble calcium and magnesium compounds is called soft water. Soft water is not easy to scum with soap, but hard water is the opposite. Natural soft water generally refers to river, river and lake (freshwater lake) water. The softened hard water refers to the softened water obtained after the content of calcium salt and magnesium salt is reduced to 1.0 ~ 50 mg/L. Although temporary hard water can be changed into soft water by boiling, it is extremely uneconomical to use this method to treat a large amount of water in industry. In daily life, we often see that the inner wall of the kettle will have scale after being used for a long time. This is because the water we use contains many inorganic salts, such as calcium and magnesium salts. These salts can't be found by naked eyes in water at room temperature. Once they are heated and boiled, many calcium and magnesium salts are precipitated in the form of carbonate, and they cling to the pot wall to form scale. We usually refer to the content of calcium and magnesium ions in water as "hardness". A hardness of 1 degree is equivalent to 10 mg of calcium oxide per liter of water. Water below 8 degrees is called soft water, water above 17 degrees is called hard water, and water between 8 and 17 degrees is called moderately hard water. Rain, snow, rivers, rivers and lakes are all soft water, while spring water, deep well water and seawater are all hard water. Some water with high content of calcium and magnesium ions does not produce scale. This is because these calcium and magnesium ions exist in the form of chloride salts, and they are soluble, so they cannot be precipitated when heated. The hardness of water has a great influence on daily life. When the hardness of water is large, washing clothes does not bubble;Living in a different place may lead to acclimatization due to the hardness of drinking water; Scaling in the pot will reduce the thermal conductivity of the pot; The scale of industrial boilers can cause explosion accidents. Therefore, the hardness of domestic and industrial water should be properly controlled. Drinking soft water (pure water) often leads to cardiovascular and cerebrovascular diseases, and drinking hard water (mineral water) often leads to kidney calculi.