Reagents Used for Softening Temporary Hardness of Water

Temporary hardness of water is caused by the presence of dissolved bicarbonates of calcium and magnesium, namely $\text{Ca(HCO}_3)_2$ and $\text{Mg(HCO}_3)_2$.

To remove temporary hardness, one common chemical method used is Clark's Method, which involves adding a calculated amount of slaked lime, $\text{Ca(OH)}_2$, to hard water.

When calcium hydroxide ($\text{Ca(OH)}_2$) is added:

1. It reacts with soluble calcium bicarbonate to precipitate insoluble calcium carbonate:
   $\text{Ca(HCO}_3)_2\text{(aq)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)}\downarrow + 2\text{H}_2\text{O(l)}$

2. It reacts with soluble magnesium bicarbonate to precipitate insoluble magnesium hydroxide and calcium carbonate:
   $\text{Mg(HCO}_3)_2\text{(aq)} + 2\text{Ca(OH)}_2\text{(aq)} \rightarrow 2\text{CaCO}_3\text{(s)}\downarrow + \text{Mg(OH)}_2\text{(s)}\downarrow + 2\text{H}_2\text{O(l)}$

The insoluble precipitates of $\text{CaCO}_3$ and $\text{Mg(OH)}_2$ are then removed by filtration, thereby softening the water.

Evaluating the given options:

• A) $\text{Ca}_3(\text{PO}_4)_2$: Insoluble in water and not used for water softening.
• B) $\text{Ca(OH)}_2$: Known as slaked lime, specifically used in Clark's process for removing temporary hardness.
• C) $\text{Na}_2\text{CO}_3$: Primarily used for removing permanent hardness of water (by precipitating soluble $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ sulfates/chlorides).
• D) $\text{NaOCl}$: Sodium hypochlorite is a disinfectant and bleaching agent, not a water softener.

Therefore, the correct reagent used for softening the temporary hardness of water is $\text{Ca(OH)}_2$.

Correct Answer: B