Hardness (water)

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The hardness of water is measured as the amount of total dissolved solids (TDS), such as Calcium sulfate or Magnesium carbonate. Hard water will leave a visible residue upon evaporation (limescale). It can also react with soaps to minimize bubble formation and to form an insoluble scum that interfers with cleaning. Hardness may be removed by distillation, by adding Sodium carbonate and Lime or by filtering the water. Filtration may be done with zeolites, an ion exchange column or a reverse osmosis system.

Synonyms and Related Terms

Wasserhärte (Deut.); dureté (Fr.); hard water; limescale; total dissolved solids (TDS)

Physical and Chemical Properties

TDS scale:

  • Soft water = 0-20 mg/l (calculated as calcium)
  • Moderately soft = 20-40 mg/l
  • Slightly hard= 40-60 mg/l
  • Moderately hard = 60-80 mg/l
  • Very hard = greater than 120 mg/l

Resources and Citations

  • Hoechst Celanese Corporation, Dictionary of Fiber & Textile Technology (older version called Man-made Fiber and Textile Dictionary, 1965), Hoechst Celanese Corporation, Charlotte NC, 1990
  • Richard C. Wolbers, Nanette T. Sterman, Chris Stavroudis, Notes for Workshop on New Methods in the Cleaning of Paintings, J.Paul Getty Trust, Los Angeles, 1990
  • Random House, Webster's Encyclopedic Unabridged Dictionary of the English Language, Grammercy Book, New York, 1997
  • The American Heritage Dictionary or Encarta, via Microsoft Bookshelf 98, Microsoft Corp., 1998

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