is the amount of dissolved calcium and magnesium in water. Hard water is high in dissolved minerals, both calcium and magnesium. When hard water is heated, solid deposits of calcium carbonate can form causing scale. This can reduce the life of equipment.

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  Measures of water hardness

  Hardness is caused by compounds of calcium and magnesium, and by a variety of other metals. General guidelines for classification of waters are: 0 to 60 mg/L (milligrams per liter) as calcium carbonate is classified as soft; 61 to 120 mg/L as moderately hard; 121 to 180 mg/L as hard; and more than 180 mg/L as very hard.

Water systems using groundwater as a source are concerned with water hardness, since as water moves through soil and rock it dissolves small amounts of naturally-occurring minerals and carries them into the groundwater supply. Water is a great solvent for calcium and magnesium, so if the minerals are present in the soil around a water-supply well, the hard water may be delivered to homes. Water hardness varies throughout the United States. In areas of the country where the water is relatively hard, industries might have to spend money to soften their water, as hard water can damage equipment.