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Hard Water

Physics and Chemistry of Water

  1. Gholam A. Kazemi

Published Online: 15 APR 2005

DOI: 10.1002/047147844X.pc251

Water Encyclopedia

Water Encyclopedia

How to Cite

Kazemi, G. A. 2005. Hard Water. Water Encyclopedia. 4:452–455.

Author Information

  1. Shahrood University of Technology, Shahrood, Iran

Publication History

  1. Published Online: 15 APR 2005


In defining hard water, first it is necessary to describe hardness, a closely related chemical property of water. Hardness is defined by some as a property of water that prevents lather formation with soap and produces scale in hot water systems; by others, in simpler terms, as the soap consuming capacity of water; and in broader terms, by Freeze and Cherry, as the “metallic ion content of water which reacts with sodium soaps to produce solid soaps or scummy residue and which reacts with negative ions, when the water is evaporated in boilers, to produce solid boiler scale.”

The ions which result in, or produce, hardness include Mg+2, Ca+2, Sr+2, Fe+2, and Mn+2, and to a lesser extent, Ba+2 and Zn+2 and other divalent ions. The first two are the most influential. There are different forms of hardness:

  1. Total Hardness

    Ca and Mg expressed as CaCO3

  2. Calcium Hardness

    Ca expressed as CaCO3

  3. Magnesium Hardness

    Mg expressed as CaCO3

  4. Temporary Hardness

    Equals alkalinity, if alkalinity is less than total hardness

  5. Permanent Hardness

    The amount of hardness greater than temporary hardness

  6. Noncarbonate Hardness

    Permanent hardness

Of the six different forms of hardness, the most important and the one that is commonly known as hardness, is total hardness.


  • hardness;
  • softening;
  • boiler;
  • colon cancer