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Soft and Hard Water

Carbonates dissolved in water

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"Hard water" is water containing a quantity of dissolved minerals. Water hardness is a function of the amount of dissolved calcium and magnesium salts, iron and aluminum.

Total hardness can be calculated as

Total hardness in ppm Carbonate  =

   2.497 ppm Calcium

    + 4.115 ppm Magnesium

    + 1.792 ppm Iron

    + 1.822 ppm Manganese

The salts occur in a variety of forms where typically calcium and magnesium bicarbonates are referred to as "temporary hardness", and sulfates and chlorides are referred to as "permanent hardness".

When hard water is heated the carbonates precipitate out of solution, forming scale on the heat surfaces.

Soap is less effective and prevented from lathering in hard water because its reacts to form calcium or magnesium salt from the organic acids of the soap.

"Soft water" is treated water where the only cation (the positively charged ion) is sodium.

Dissolved Calcium and Magnesium
Water Hardness Classification1)Milligrams per Liter
(mg/L)
Grains per US Gallon
(gpg)
Soft 0 - 60 0 - 3.5
Moderate 61 - 120 3.5 - 7
Hard 121 - 180 7 - 10.5
Very Hard > 180 > 10.5

1) United States Geological Survey

  • 1 mg/L = 1 ppm
  • 1 ppm = 0.058 grains/US gallon
  • 1 ppm = 0.07 Clark degrees
  • 1 ppm = 0.1 French degrees

Hard water can be softened by passing it over an ion exchange resin.

  • zeolite processes for home water softeners
  • soda ash process for larger water softeners

 

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  • en: water treatment soft hard
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