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Hardness is measure of polyvalent cations (ions with a charge greater than +1) in water. Hardness generally represents the concentration of calcium (Ca2+) and magnesium (Mg2+) ions, because these are the most common polyvalent cations. Other ions, such as iron (Fe2+) and manganese (Mn2+), may also contribute to the hardness of water, but are generally present in much lower concentrations. Waters with high hardness values are referred to as "hard," while those with low hardness values are "soft".

Hardness affects the amount of soap that is needed to produce foam or lather. Hard water requires more soap, because the calcium and magnesium ions form complexes with soap, preventing the soap from sudsing. Hard water can also leave a film on hair, fabrics, and glassware. Hardness of the water is very important in industrial uses, because it forms scale in heat exchange equipment, boilers, and pipe lines. Some hardness is needed in plumbing systems to prevent corrosion of pipes.

Hardness mitigates metals toxicity, because Ca2+ and Mg2+ help keep fish from absorbing metals such as lead, arsenic, and cadmium into their bloodstream through their gills. The greater the hardness, the harder it is for toxic metals to be absorbed


Water hardness is commonly defined as the sum of the polyvalent cations dissolved in the water. The most common such cations are calcium and magnesium. Hardness is usually reported as an equivalent quantity of calcium carbonate (CaCO3). Generally, waters are classified according to degree of hardness. The hardness of your water will be reported in milligrams per liter (mg/l). Water hardness is classified by the U.S. Department of Interior and the Water Quality Association as follows:

Classification  mg/l or ppm                        

Soft                          0 - 17.1                                         
Slightly hard            17.1 - 60                
Moderately hard      60 - 120                
Hard                       120 - 180                                       
Very hard               180 & over             

Hardness is primarily a function of the geology of the area with which the surface water is associated. Waters underlain by limestone are prone to hard water because rainfall, which is naturally acidic because it contains carbon dioxide gas, continually dissolves the rock and carries the dissolved cations to the water system.