Understanding Water Hardness and Its Removal

Water hardness is caused by the presence of dissolved minerals, primarily calcium and magnesium ions. There are two types of hardness: temporary hardness and permanent hardness.

• Temporary Hardness: Caused by the presence of dissolved bicarbonates of calcium and magnesium ($\text{Ca(HCO}_3)_2$ and $\text{Mg(HCO}_3)_2$). This type of hardness can be removed by simple boiling, which decomposes the bicarbonates into insoluble carbonates, which then settle down.
• Permanent Hardness: Caused by the presence of dissolved chlorides and sulfates of calcium and magnesium ($\text{CaCl}_2$, $\text{CaSO}_4$, $\text{MgCl}_2$, $\text{MgSO}_4$). This type of hardness cannot be removed by boiling. Special chemical methods are required to remove permanent hardness.

Removing Permanent Hardness of Water

Several methods can be used to remove permanent hardness. One common and effective method involves using washing soda.

Washing Soda Method:

Washing soda is the common name for sodium carbonate ($\text{Na}_2\text{CO}_3$). When washing soda is added to hard water containing calcium and magnesium sulfates or chlorides, it reacts with the metal ions ($\text{Ca}^{2+}$ and $\text{Mg}^{2+}$) to form insoluble calcium carbonate ($\text{CaCO}_3$) and magnesium carbonate ($\text{MgCO}_3$). These precipitates can then be filtered out, effectively removing the permanent hardness.

The chemical reactions involved are:

Reaction with Calcium Chloride:

$\text{CaCl}_2\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} + 2\text{NaCl(aq)}$

Reaction with Magnesium Sulfate:

$\text{MgSO}_4\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow \text{MgCO}_3\text{(s)} + \text{Na}_2\text{SO}_4\text{(aq)}$

In both reactions, the calcium and magnesium ions are removed from the solution as solid precipitates, leaving behind soluble sodium salts which do not cause hardness.

Analysis of Options for Hardness Removal

Let's examine why other options are typically not used for removing permanent hardness:

• Baking soda ($\text{NaHCO}_3$): Sodium bicarbonate is primarily used as leavening agent in baking, antacid, etc. While it can react with some metal ions, it is not as effective or commonly used as washing soda for large-scale permanent hardness removal.
• Lime ($\text{CaO}$ or $\text{Ca(OH)}_2$): Lime is primarily used in the Clark's process to remove temporary hardness by converting soluble bicarbonates into insoluble carbonates. It can also be used in combination with soda ash (washing soda) in the lime-soda process to remove both temporary and permanent hardness, but lime alone is not effective for permanent hardness removal from sulfates and chlorides.
• Bleaching powder ($\text{CaOCl}_2$): Bleaching powder is calcium hypochlorite and is primarily used as a disinfectant to kill bacteria in water due to its chlorine content. It does not react with calcium or magnesium ions to remove hardness.

Therefore, washing soda ($\text{Na}_2\text{CO}_3$) is the substance among the given options that is commonly used specifically for removing the permanent hardness of water.

Revision Table: Water Hardness Removal Methods

Additional Information on Water Softening

Water softening is the process of removing the ions that cause water hardness, primarily calcium and magnesium ions. Besides the washing soda method, other techniques are employed:

• Ion-Exchange Process: This involves using zeolite or resin columns that exchange sodium ions from the resin with calcium and magnesium ions from the water. This is a very effective method for removing both temporary and permanent hardness, producing very soft water.
• Calgon Process: This method uses sodium hexametaphosphate (Calgon) which forms soluble complex ions with $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions, preventing them from forming precipitates and effectively softening the water.
• Reverse Osmosis: This is a membrane process that removes a wide range of dissolved ions, including those causing hardness, by forcing water through a semi-permeable membrane under pressure.

Understanding these different methods provides a broader perspective on water treatment for hardness reduction.