Water becomes permanently hard due to the presence of dissolved chloride and sulphate salts of calcium and magnesium. These salts remain dissolved even after boiling, unlike the salts that cause temporary hardness.
Here's a breakdown:
-
Permanent Hardness Explained: Permanent hardness in water is caused by the presence of calcium and magnesium chlorides (e.g., CaCl₂) and sulphates (e.g., MgSO₄). These salts do not precipitate out of the water when it is boiled.
-
Why Boiling Doesn't Work: Unlike temporary hardness, which is caused by bicarbonates of calcium and magnesium that decompose upon heating, chloride and sulphate salts are thermally stable and remain in solution even at boiling temperatures.
-
Removal Methods: Permanent hardness can be removed through processes like:
- Ion Exchange Resins: This involves using cation exchange resins to replace the calcium and magnesium ions with sodium ions, effectively softening the water. The reference mentions this.
- Chemical Treatment: Adding washing soda (sodium carbonate) precipitates the calcium and magnesium as carbonates, which can then be filtered out.
-
Acceptable Limits: While calcium and magnesium are essential minerals, their presence in high concentrations makes water hard. Recommended safety limits in drinking water typically fall in these ranges:
- Calcium: 40 - 80 ppm (parts per million)
- Magnesium: 20 - 30 ppm
In summary, water becomes permanently hard when it contains dissolved calcium and magnesium chlorides and sulphates, which boiling cannot remove. Specialized water softening techniques like ion exchange or chemical treatment are required to address this type of hardness.
