What Is Hardness?

Hard water and soft water are terms that have no exact meaning because water considered hard in one region might be considered soft by inhabitants in another region. Hardness is usually associated with what happens when soap is used. Soap does not clean as efficiently in hard water. 

It leaves insoluble residues in bathtubs, sinks and clothing. In addition, hard water causes scale to incrust water heaters, boilers and pipes thereby reducing their capacity and heat-transfer properties. 

Thus, it is a technical and economic problem. (See Section 3 : Water Softener).
Hardness is commonly reported in units milligrams per litre as CaCO, although other terms are also used, as follows:
e.g. grains per gallon (U.S.) - gpg
1 gpg (U.S.) = 17.1 mg/L TOTAL HARDNESS as CaCO
Grains per gallon, an ancient concept, was originally defined as the degree of hardness produced by a grain of chalk in a gallon of water.

For the most part, hardness depends on the concentration of calcium and magnesium. Hardness is usually expressed in terms of the equivalent quantity of calcium carbonate (CaCO). There are no distinctly defined levels for what constitutes a hard or soft water supply. However, a general scale of hardness (as CaCO) has been provided by the Water Quality Association as follows:
mg/L Description

0 - 8.5 Very Soft
9 - 30 Soft
31 - 60 Slightly Hard
60 - 120 Moderately Hard
121 - 185 Hard
Over 185 Very Hard

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