Boiling softens hard water by causing dissolved minerals to precipitate out of the solution. This is specifically effective for temporary hardness caused by the presence of calcium and magnesium hydrogencarbonates.
Understanding Hard Water
Hard water contains high concentrations of dissolved minerals, particularly calcium (Ca2+) and magnesium (Mg2+) ions. These ions interfere with the action of soaps and detergents, causing scaling in pipes and appliances. Hardness can be temporary or permanent.
The Boiling Process: Removing Temporary Hardness
The process of boiling only addresses temporary hardness, which is due to the presence of calcium hydrogencarbonate (Ca(HCO3)2) and magnesium hydrogencarbonate (Mg(HCO3)2) dissolved in the water. When hard water containing these compounds is boiled:
- Decomposition: The heat causes the hydrogencarbonates to decompose.
- Precipitation: This decomposition results in the formation of insoluble calcium carbonate (CaCO3) or magnesium carbonate (MgCO3), which then precipitate out of the water as a solid. The reference states: "boiling the water causes the precipitation of solid calcium carbonate or solid magnesium carbonate."
- Softening: Because the calcium and magnesium ions are now removed from the water, the water becomes softer. As the reference notes: "This removes the calcium ions or magnesium ions from the water, and so removes the hardness."
The chemical equations representing these reactions are:
Ca(HCO3)2 (aq) → CaCO3 (s) + H2O (l) + CO2 (g)
Mg(HCO3)2 (aq) → MgCO3 (s) + H2O (l) + CO2 (g)
Practical Implications
- Scale Formation: The white solid you often see forming in kettles or pots when boiling hard water is primarily calcium carbonate (scale).
- Temporary vs. Permanent Hardness: Boiling does not remove permanent hardness, which is caused by other dissolved calcium and magnesium salts (like calcium sulfate or magnesium chloride) that do not decompose upon heating.
| Feature | Temporary Hardness | Permanent Hardness |
|---|---|---|
| Cause | Calcium and Magnesium Hydrogencarbonates | Other Calcium and Magnesium Salts (e.g., sulfates) |
| Removal by Boiling | Yes | No |
| Resulting precipitate | Calcium and Magnesium Carbonates | N/A (Requires chemical treatment) |
In conclusion, boiling softens hard water by precipitating out calcium and magnesium carbonates, but this only works for temporary hardness.
