Hard Water Definition

What Hard Water Is and What It Does

water dripping from tap
Hard water is simply water that contains high levels of calcium and magnesium ions. Tim Graham/Getty Images

Hard water is water that contains high amounts of Ca2+ and/or Mg2+. Sometimes Mn2+ and other multivalent cations are included in the measure of hardness. Note water may contains minerals and yet not be considered hard, by this definition. Hard water occurs naturally under condition where water percolates through calcium carbonates or magnesium carbonates, such as chalk or limestone.

Evaluating How Hard Water Is

According to the USGS, the hardness of water is determined based on the concentration of dissolved multivalent cations:

  • soft water - 0 to 60 mg/L (milligrams per liter) as calcium carbonate
  • moderately hard water - 61 to 120 mg/L
  • hard water - 121 to 180 mg/L
  • very hard water - more than 180 mg/L

Hard Water Effects

Both positive and negative effects of hard water are known:

  • Hard water may offer health benefits as drinking water, compared with soft water. Drinking hard water and beverages made using hard water can contribute to dietary requirements for calcium and magnesium.
  • Soap is a less effective cleaner in hard water. Hard water makes it harder to rinse soap, plus it forms a curd or soap scum. Detergent is also affected by the dissolved minerals in hard water, but not to the same extent as soap. More soap or detergent are required to clean clothes and other items using hard water compared with soft water. Hair washed in hard water may appear dull and feel stiff from residue. Clothes washed in hard water may develop a yellowish or gray discoloration and may feel stiff.
  • Soap residue left on skin from bathing in hard water can trap bacteria on the skin surface and disrupt the normal balance of microflora. Because the residue inhibits the ability of skin to return to its slightly acidic pH, irritation may occur.
  • Hard water can leave behind water spots on dishes, windows, and other surfaces.
  • Minerals in hard water can deposit in pipes and on surfaces forming scale. This can clog pipes over time and decrease water heater efficiency. One positive aspect of scale is that it forms a barrier between pipes and water, limiting leaching of solder and metals into water.
  • The electrolytes in hard water can lead to galvanic corrosion, which is when one metal corrodes when in contact with another metal in the presence of ions.

Temporary and Permanent Hard Water

Temporary hardness is characterized by dissolved bicarbonate minerals (calcium bicarbonate and magnesium bicarbonate) that yield calcium and magnesium cations (Ca2+, Mg2+) and carbonate and bicarbonate anions (CO32−, HCO3). This type of water hardness may be reduced by adding calcium hydroxide to the water or by boiling it.

Permanent hardness is generally associated with calcium sulfate and/or magnesium sulfates in the water, which will not precipitate when the water is boiled. Total permanent hardness is the sum of the calcium hardness plus the magnesium hardness. This type of hard water may be softened by using an ion exchange column or water softener.

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Helmenstine, Anne Marie, Ph.D. "Hard Water Definition." ThoughtCo, Feb. 10, 2017, thoughtco.com/definition-of-hard-water-604526. Helmenstine, Anne Marie, Ph.D. (2017, February 10). Hard Water Definition. Retrieved from https://www.thoughtco.com/definition-of-hard-water-604526 Helmenstine, Anne Marie, Ph.D. "Hard Water Definition." ThoughtCo. https://www.thoughtco.com/definition-of-hard-water-604526 (accessed November 22, 2017).