Why hardness is generally expressed as calcium carbonate equivalent?
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The "hardness"* of water is often expressed as a calcium carbonate equivalent because calcium carbonate is one of the most common solutes in hard water, which precipitates much of the mass of soap added to the water as a calcium salt with the anions in soap.
*This is not what is meant ordinarily by the word hardness**, but rather a measure of the tendency of some natural water supplies to form a precipitate with soaps added to the water, so that more soap is required to accomplish washing in such waters than is needed in "soft" water: Water, such as distilled water or natural sodic waters, that contain little or no polyvalent cations, such as calcium, magnesium, or aluminum.
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