Hardness refers to some metal ions which are easy to form precipitation in water. They are divalent or over divalent ions (such as Ca2+, Mg2+, Fe3+, Mn2+). In natural water, hardness is mainly formed by calcium and magnesium ions, so hardness is generally considered to be the amount of these two ions. Calcium salts include: calcium bicarbonate, calcium carbonate, calcium sulfate, calcium chloride; magnesium salts include: magnesium bicarbonate, magnesium carbonate, magnesium sulfate, magnesium chloride. Calcium salt part is called calcium hardness, and magnesium salt part is called magnesium hardness. The total hardness is equal to the sum of the two.

(a) Hardness classification: Hardness can be divided into carbonate hardness and non-carbonate hardness according to the anion species in water.

1. Carbonate hardness refers to the content of bicarbonate and carbonate of calcium and magnesium in water. natural water

Medium carbonate is very few, so the carbonate hardness is regarded as the bicarbonate hardness of calcium and magnesium. The hardness of this kind of salt precipitates from the solution when water boils, so it is also called temporary hardness.

2. Non-carbonate hardness refers to the content of sulfate and chloride of calcium and magnesium in water. Because this hardness cannot precipitate when water boils, it is also called permanent hardness.

When the content of HCO 3-in water is less than the total calcium and magnesium in water, the hardness of water has carbonate hardness and non-carbonate hardness; when the content of HCO 3-in water is greater than the total calcium and magnesium in water, the hardness of water has carbonate hardness and excess alkalinity (sometimes called negative hardness), which is called negative hard water.