Ca and Mg expressed as CaCO3
Physics and Chemistry of Water
Published Online: 15 APR 2005
Copyright © 2005 by John Wiley & Sons, Inc.
How to Cite
Kazemi, G. A. 2005. Hard Water. Water Encyclopedia. 4:452–455.
- 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:
- Total Hardness
- Calcium Hardness
Ca expressed as CaCO3
- Magnesium Hardness
Mg expressed as CaCO3
- Temporary Hardness
Equals alkalinity, if alkalinity is less than total hardness
- Permanent Hardness
The amount of hardness greater than temporary hardness
- Noncarbonate Hardness
Of the six different forms of hardness, the most important and the one that is commonly known as hardness, is total hardness.
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