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- Hard water is water which contains minerals like calcium carbonate, magnesium carbonate, bicarbonates, sulfates, or chlorides, because it has been exposed to rocks or rocky soils. Hard water can corrode, discolor, or deposit its minerals, in and around materials such as water pipes.
- Total concentration of calcium and magnesium ions, expressed as the equivalent concentration (mg/L) of calcium carbonate. "Hard" water is water that contains lots of calcium carbonate and other minerals.