Temporary Hardness Explained
Water hardness refers to the concentration of dissolved minerals, primarily calcium (Ca) and magnesium (Mg) ions. Hardness is categorized into two main types: temporary and permanent.
Understanding Temporary Water Hardness
Temporary hardness is caused by the presence of dissolved bicarbonates of calcium and magnesium.
- Calcium Bicarbonate: \(Ca(HCO3)2\)
- Magnesium Bicarbonate: \(Mg(HCO3)2\)
This type of hardness is called "temporary" because it can be easily removed by simply boiling the water. Boiling causes the soluble bicarbonates to convert into insoluble carbonates, which precipitate out of the water.
The chemical reaction for magnesium bicarbonate upon heating is:
\(Mg(HCO3)2 (aq) + Heat → MgCO3 (s) + H2O (l) + CO2 (g)\)
The resulting magnesium carbonate (\(MgCO3\)) is a solid precipitate (scale) and is removed, thus softening the water.
Comparing Hardness Types
Permanent hardness, on the other hand, is caused by the presence of dissolved sulphates, chlorides, and nitrates of calcium and magnesium.
- Sulphates: e.g., \(CaSO4\), \(MgSO4\)
- Chlorides: e.g., \(CaCl2\), \(MgCl2\)
These compounds do not precipitate upon boiling, making this type of hardness difficult to remove by simple heating.
Identifying the Salt Responsible
The question asks which salt is responsible for temporary hardness. Based on the explanations above:
- Sulphates of calcium (Option 1) cause permanent hardness.
- Chlorides of magnesium (Option 2) cause permanent hardness.
- Bicarbonates of magnesium (Option 3) cause temporary hardness.
- Sulphates of magnesium (Option 4) cause permanent hardness.
Therefore, the salt responsible for the temporary hardness of water among the given options is bicarbonates of magnesium.
| Type of Hardness | Causing Salts | Removal Method |
|---|---|---|
| Temporary | Bicarbonates (e.g., \(Ca(HCO3)2\), \(Mg(HCO3)2\)) | Boiling |
| Permanent | Sulphates, Chlorides (e.g., \(CaSO4\), \(CaCl2\), \(MgSO4\), \(MgCl2\)) | Chemical Treatment (e.g., Ion Exchange) |
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