Water hardness is a measure of the amount of calcium and magnesium salts in
water. Calcium and magnesium enter water mainly through the weathering of rocks.
The more calcium and magnesium in water, the harder the water. Water hardness is
usually expressed in milligrams per liter (mg/l) of dissolved calcium and
magnesium carbonate.
Source: GreenFacts
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The term "hardness" comes from the fact that it is hard to get soapsuds from
soap or detergents in hard water. This happens because calcium and magnesium
react strongly with negatively-charged chemicals like soap to form insoluble
compounds. As a result, hard water can reduce the effectiveness of the cleaning
process.
