Removing Permanent Hardness of Water

Water hardness refers to the concentration of multivalent cations, primarily calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)) ions. It's classified into two types:

• Temporary Hardness: Caused by the presence of bicarbonates of calcium and magnesium. It can be removed by boiling.
• Permanent Hardness: Caused by the presence of soluble chlorides and sulfates of calcium and magnesium. This type cannot be removed by boiling.

Understanding Permanent Hardness Issues

Permanent hardness causes problems like:

• Formation of scale in boilers and pipes, reducing efficiency and causing damage.
• Reduced lathering of soap, requiring more soap for washing.

Methods to Remove Permanent Hardness

Permanent hardness is typically removed using chemical treatments that precipitate the dissolved hardness-causing ions (\(Ca^{2+}\) and \(Mg^{2+}\)). Let's analyze the given options:

Analysis of Options:

• Bleaching Powder (Calcium Hypochlorite, \(CaOCl_2\)): Primarily used as a disinfectant and bleaching agent. It is not used for removing water hardness.
• Sodium Hydroxide (Caustic Soda, \(NaOH\)): While it can precipitate certain ions (like \(Mg^{2+}\) as \(Mg(OH)_2\)), it's not the standard or most common reagent specifically designated for removing permanent hardness in typical water treatment scenarios compared to washing soda. It's often used for pH adjustment.
• Baking Soda (Sodium Bicarbonate, \(NaHCO_3\)): Mainly helps in removing temporary hardness and acts as a mild base. It does not effectively precipitate the sulfates or chlorides responsible for permanent hardness.
• Washing Soda (Sodium Carbonate, \(Na_2CO_3\)): This is the standard chemical used to remove permanent hardness. It reacts with the soluble calcium and magnesium salts to form insoluble carbonate precipitates.

Washing Soda: The Solution for Permanent Hardness

Washing soda, chemically known as Sodium Carbonate (\(Na_2CO_3\)), effectively removes permanent hardness by converting soluble hardness salts into insoluble precipitates. The carbonate ions (\(CO_3^{2-}\)) from washing soda react with calcium (\(Ca^{2+}\)) and magnesium (\(Mg^{2+}\)) ions.

Chemical Reactions:

When washing soda is added to hard water containing calcium and magnesium sulfates or chlorides, the following reactions occur:

• Reaction with Calcium Sulfate (\(CaSO_4\)): \(Na_2CO_3 \text{ (aq)} + CaSO_4 \text{ (aq)} \rightarrow CaCO_3 \text{ (s)} \downarrow + Na_2SO_4 \text{ (aq)}\)
• Reaction with Magnesium Sulfate (\(MgSO_4\)): \(Na_2CO_3 \text{ (aq)} + MgSO_4 \text{ (aq)} \rightarrow MgCO_3 \text{ (s)} \downarrow + Na_2SO_4 \text{ (aq)}\)
• Similarly, for chlorides: \(Na_2CO_3 \text{ (aq)} + CaCl_2 \text{ (aq)} \rightarrow CaCO_3 \text{ (s)} \downarrow + 2 NaCl \text{ (aq)}\) \(Na_2CO_3 \text{ (aq)} + MgCl_2 \text{ (aq)} \rightarrow MgCO_3 \text{ (s)} \downarrow + 2 NaCl \text{ (aq)}\)

The resulting precipitates, Calcium Carbonate (\(CaCO_3\)) and Magnesium Carbonate (\(MgCO_3\)), are insoluble in water and can be removed by filtration. This process effectively reduces the permanent hardness of the water.

Summary Table

Therefore, washing soda is the correct chemical agent used to remove the permanent hardness of water.