What is hardness?

Document ID

Document ID TE280

Published Date

Published Date 09/09/2020
What is hardness?
Definition of hardness
Hardness in water refers to specific minerals that consume soap and cause scaling in water heaters and boilers. The more minerals, the harder the water. Soft water refers to the absence of these minerals. The term hardness comes from an expression of how difficult or "hard" it is to wash clothes with the water. When soap is mixed with hard water, these minerals combine with the soap and form a precipitate, or a solid. This decreases the cleaning efficiency of the soap and forms soap scum. As more soap is added, solids continue to form until the minerals are depleted. When the minerals are no longer available, the soap forms a lather and works as a cleaning agent. The minerals that precipitate with soap are polyvalent cations such as calcium, magnesium, iron, manganese, and zinc. The concentration of calcium and magnesium in natural waters generally far exceeds that of any other polyvalent cation. Therefore, hardness is generally considered to be the concentration of calcium and magnesium in water.

Carbonate and Non-Carbonate Hardness:
Hardness can be classified as carbonate and non-carbonate hardness. Carbonate hardness refers to calcium and magnesium bicarbonate. When calcium bicarbonate is heated, solid calcium carbonate forms. This is the primary cause of scale formation in water heaters and boilers. Non-carbonate hardness is caused primarily by calcium and magnesium nitrates, chlorides, and sulfates.

Hardness is removed from water systems by precipitation or ion exchange. The treatment method varies depending on the relative amounts of carbonate vs. non-carbonate hardness. The amount of carbonate vs. non-carbonate hardness can be found by measuring alkalinity. If the alkalinity is equal to or greater than the hardness, all of the hardness is carbonate. Any excess hardness is non-carbonate hardness.

Hardness is typically reported in terms of mg/L as CaCO 3 or gpg as CaCO 3. Because alkalinity is also reported as CaCO 3, the results of the two tests can be compared directly.


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