Water that does not contain or contains less soluble calcium and magnesium compounds is called soft water. Soft water is less likely to produce scum with soap, while hard water is the opposite. Natural soft water generally refers to river water, river water, and freshwater lake water. Softened hard water refers to the softened water obtained by reducing the content of calcium and magnesium salts to 1.0-50 mg/L. Although boiling can turn temporarily hard water into soft water, it is extremely uneconomical to use this method to treat large amounts of water in industry.

In daily life, we often see the formation of scale on the inner wall of a kettle after prolonged use. This is because the water we use contains many inorganic salts, such as calcium and magnesium salts. These salts cannot be detected by the naked eye in water at room temperature. Once they are heated and boiled, many calcium and magnesium salts precipitate in the form of carbonates, and they adhere tightly to the pot wall to form scale. We usually use the indicator of "hardness" to represent the content of calcium and magnesium ions in water. A hardness of 1 degree is equivalent to 10 milligrams 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, lakes are all soft water, while spring water, deep well water, and seawater are hard water.
Some water with high calcium and magnesium ion content does not show scale formation because these calcium and magnesium ions exist in the form of chloride salts, which are soluble and cannot precipitate out when heated.
The hardness of water has a significant impact on daily life. When the hardness of water is high, washing clothes does not produce bubbles; Living in a different place may result in symptoms of discomfort due to the hardness of drinking water; The accumulation of scale inside the pot will decrease its thermal conductivity; Scale on industrial boilers can cause explosion accidents. Therefore, the hardness of water should be appropriately controlled for both domestic and industrial use. Generally speaking, soft water is often used in daily life, such as bathing and washing clothes. Not for drinking, contains too few minerals.