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Difficulty: MediumHardness of Water: Causes, Types, and Removal

During industrial water treatment, temporary hardness caused by dissolved calcium hydrogencarbonate is removed chemically via Clark's process by adding a controlled quantity of slaked lime, Ca(OH)2\text{Ca(OH)}_2. Which balanced chemical equation correctly represents this water-softening reaction?

  1. Ca(HCO3)2(aq)+Ca(OH)2(aq)→2CaCO3(s)+2H2O(l)\text{Ca(HCO}_3)_2\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)} + 2\text{H}_2\text{O(l)}Answer
  2. B
    Ca(HCO3)2(aq)+Ca(OH)2(aq)→CaCO3(s)+CO2(g)+2H2O(l)\text{Ca(HCO}_3)_2\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}
  3. C
    CaSO4(aq)+Ca(OH)2(aq)→2CaCO3(s)+H2SO4(aq)\text{CaSO}_4\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)} + \text{H}_2\text{SO}_4\text{(aq)}
  4. D
    Ca(HCO3)2(aq)+CaO(s)→CaCO3(s)+H2O(l)\text{Ca(HCO}_3)_2\text{(aq)} + \text{CaO(s)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}

Answer

Ca(HCO3)2(aq)+Ca(OH)2(aq)→2CaCO3(s)+2H2O(l)\text{Ca(HCO}_3)_2\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)} + 2\text{H}_2\text{O(l)}
The equation Ca(HCO3)2(aq)+Ca(OH)2(aq)→2CaCO3(s)+2H2O(l)\text{Ca(HCO}_3)_2\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)} + 2\text{H}_2\text{O(l)} represents Clark's process. Slaked lime provides hydroxide ions that neutralize the hydrogencarbonate ions in temporary hard water, precipitating all calcium as insoluble calcium trioxocarbonate(IV), leaving soft water.

Step-by-Step Solution

1
Identify the chemical cause of temporary hardness in the water sample
Temporary hardness is caused by dissolved calcium hydrogencarbonate, Ca(HCO3)2(aq)\text{Ca(HCO}_3)_2\text{(aq)}.
Hydrogencarbonate salts of calcium and magnesium decompose or precipitate when treated with appropriate alkaline agents.
2
Apply the principles of Clark's process for temporary hardness removal
Clark's process involves adding a calculated amount of slaked lime, Ca(OH)2(aq)\text{Ca(OH)}_2\text{(aq)}, which supplies hydroxide ions to convert hydrogencarbonate ions into insoluble trioxocarbonate(IV) ions.
Adding excess lime would re-introduce calcium ions and cause artificial hardness, so a stoichiometric amount is used.
3
Balance the precipitation chemical equation
Ca(HCO3)2(aq)+Ca(OH)2(aq)→2CaCO3(s)+2H2O(l)\text{Ca(HCO}_3)_2\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)} + 2\text{H}_2\text{O(l)}
One mole of calcium hydrogencarbonate reacts with one mole of calcium hydroxide to produce two moles of insoluble calcium trioxocarbonate(IV) and two moles of water.

Key Concept

Clark's process (slaked lime precipitation) for removal of temporary water hardness
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Topics

Hardness of Water: Causes, Types, and Removal

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