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Water hardness means water has dissolved minerals, mainly calcium and magnesium. Hard water can be temporary or permanent.
Permanent hardness is caused by calcium sulfate (CaSO₄) or magnesium sulfate (MgSO₄) in water. Unlike temporary hardness, permanent hardness cannot be removed by boiling, because these sulfates stay dissolved.
If water has permanent hardness, it does not make proper lather with soap. Instead, soap forms scum, which is a sticky residue. To remove permanent hardness, special methods are used, like ion-exchange softeners, lime treatment, or chemical precipitation.
Temporary hardness, in comparison, is caused by calcium bicarbonate (Ca(HCO₃)₂) or magnesium bicarbonate (Mg(HCO₃)₂). Boiling removes this type of hardness because the bicarbonates turn into insoluble carbonates and gas:
Ca(HCO₃)₂ → CaCO₃ ↓ + CO₂ + H₂O
Permanent hardness is an important concept in chemistry, especially in water treatment, household use, and industrial applications. Knowing the cause of permanent hardness helps us understand why some water is hard to use for laundry, cooking, or industrial machines without treatment.
In simple terms, the main cause of permanent hardness is calcium sulfate, and it stays in water even after heating.
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