Heating water that has temporary hardness reduces the hardness by causing dissolved bicarbonates to decompose and precipitate out as insoluble carbonates.
Understanding Temporary Hardness
Temporary hardness in water is primarily caused by the presence of dissolved bicarbonate salts of calcium and magnesium, specifically calcium bicarbonate (Ca(HCO₃)₂) and magnesium bicarbonate (Mg(HCO₃)₂). These salts are formed when rainwater, containing dissolved carbon dioxide, flows over limestone or dolomite rock, dissolving the calcium and magnesium carbonates present in the rock.
The Effect of Temperature: Decomposition and Precipitation
Here's how increasing the temperature affects temporary hardness:
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Decomposition of Bicarbonates: When water containing calcium bicarbonate or magnesium bicarbonate is heated, the bicarbonate ions (HCO₃⁻) decompose. The chemical equation for this reaction is:
Ca(HCO₃)₂ (aq) → CaCO₃ (s) + H₂O (l) + CO₂ (g)
Mg(HCO₃)₂ (aq) → MgCO₃ (s) + H₂O (l) + CO₂ (g)This essentially means the bicarbonate breaks down into carbonate, water, and carbon dioxide. The released carbon dioxide escapes as a gas.
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Formation of Insoluble Carbonates: The carbonate ions (CO₃²⁻) then react with the calcium (Ca²⁺) and magnesium (Mg²⁺) ions present in the water to form insoluble calcium carbonate (CaCO₃) and magnesium carbonate (MgCO₃).
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Precipitation: Because calcium carbonate and magnesium carbonate are largely insoluble in water, they precipitate out of the solution as a solid. This manifests as a white, scaly deposit often seen in kettles, boilers, and pipes that heat hard water. This process effectively removes the calcium and magnesium ions from the water, thereby reducing its hardness.
Summary
In summary, increasing the temperature of water containing temporary hardness leads to the decomposition of bicarbonates and the subsequent precipitation of insoluble carbonates, thus reducing the water's hardness. This is why boiling water is a simple method for softening temporarily hard water.
