Understanding Water Hardness and Its Removal

Water hardness is caused by the presence of dissolved minerals, primarily calcium and magnesium ions. Hard water can cause issues like scaling in pipes and boilers, reduced lathering of soap, and affecting the taste of water.

There are two types of water hardness:

• Temporary Hardness: Caused by dissolved bicarbonates of calcium and magnesium \(\left(\text{Ca(HCO}_3\text{)}_2, \text{ Mg(HCO}_3\text{)}_2\right)\). This can be removed by simple boiling.
• Permanent Hardness: Caused by dissolved chlorides and sulfates of calcium and magnesium \(\left(\text{CaCl}_2, \text{ MgCl}_2, \text{ CaSO}_4, \text{ MgSO}_4\right)\). This type of hardness cannot be removed by boiling. Special methods are required to remove permanent hardness.

Methods for Removing Permanent Hardness

Permanent hardness needs specific treatment methods to remove the dissolved calcium and magnesium ions. Let's examine the given options:

Treatment with Washing Soda

Washing soda \(\left(\text{Na}_2\text{CO}_3\right)\) is an effective method to remove permanent hardness. When washing soda is added to hard water, it reacts with the dissolved calcium and magnesium sulfates and chlorides to form insoluble calcium carbonate \(\left(\text{CaCO}_3\right)\) and magnesium carbonate \(\left(\text{MgCO}_3\right)\).

The reactions are as follows:

• \(\text{CaSO}_4\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} \downarrow + \text{Na}_2\text{SO}_4\text{(aq)}\)
• \(\text{MgSO}_4\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow \text{MgCO}_3\text{(s)} \downarrow + \text{Na}_2\text{SO}_4\text{(aq)}\)
• \(\text{CaCl}_2\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow \text{CaCO}_3\text{(s)} \downarrow + 2\text{NaCl(aq)}\)
• \(\text{MgCl}_2\text{(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow \text{MgCO}_3\text{(s)} \downarrow + 2\text{NaCl(aq)}\)

The precipitated carbonates are then filtered out, thus removing the calcium and magnesium ions that cause permanent hardness. So, this method can remove permanent hardness.

Calgon’s Method

Calgon's method uses sodium hexametaphosphate \(\left(\text{Na}_6[\text{P}_6\text{O}_{18}]\right)\), commonly known as Calgon. This compound acts as a sequestering agent, meaning it binds with the calcium and magnesium ions in the hard water to form soluble complex ions. These complex ions do not cause hardness.

The reaction is typically represented as:

• \(2\text{Ca}^{2+}\text{(aq)} + [\text{P}_6\text{O}_{18}]^{6-}\text{(aq)} \rightarrow [\text{Ca}_2\text{P}_6\text{O}_{18}]^{2-}\text{(aq)}\)

Similar reactions occur with magnesium ions. By forming these stable, soluble complexes, the calcium and magnesium ions are removed from the water's ability to react with soap or precipitate as scale. Thus, this method effectively removes permanent hardness.

Boiling

Boiling is a simple method, but it only removes temporary hardness. When water containing temporary hardness (bicarbonates) is boiled, the bicarbonates decompose into insoluble carbonates, which precipitate out.

The reactions are:

• \(\text{Ca(HCO}_3\text{)}_2\text{(aq)} \xrightarrow{\text{Heat}} \text{CaCO}_3\text{(s)} \downarrow + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)
• \(\text{Mg(HCO}_3\text{)}_2\text{(aq)} \xrightarrow{\text{Heat}} \text{MgCO}_3\text{(s)} \downarrow + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}\)

However, the sulfates and chlorides of calcium and magnesium that cause permanent hardness are stable and do not precipitate upon boiling. Therefore, boiling cannot remove permanent hardness.

Ion Exchange Method

The ion exchange method is a highly effective way to remove both temporary and permanent hardness, making water essentially demineralized. This method uses ion exchange resins, which are typically organic polymers with charged functional groups.

In a common type of resin (like zeolite or synthetic resins), calcium and magnesium ions in the hard water are exchanged for sodium ions (or sometimes hydrogen ions) that are initially bound to the resin.

• \(\text{Ca}^{2+}\text{(aq)} + \text{Na}_2\text{R(s)} \rightarrow \text{CaR(s)} + 2\text{Na}^+\text{(aq)}\)
• \(\text{Mg}^{2+}\text{(aq)} + \text{Na}_2\text{R(s)} \rightarrow \text{MgR(s)} + 2\text{Na}^+\text{(aq)}\)

Here, R represents the anionic part of the resin. As water passes through the resin bed, calcium and magnesium ions are removed from the water and replaced by sodium ions, which do not cause hardness. This method effectively removes permanent hardness.

Conclusion

Based on the analysis of each method, boiling is the only process listed that specifically does not remove permanent hardness of water. It is effective only for temporary hardness.

Revision Table: Water Hardness Removal

Here's a quick summary of the methods and the type of hardness they remove:

Additional Information on Water Hardness Treatment

Understanding water hardness and its treatment methods is important in chemistry and environmental science. Permanent hardness, caused by sulfates and chlorides of calcium and magnesium, requires chemical methods or ion exchange to effectively remove these dissolved salts.

Other methods not listed in the options but used for water softening include the Lime-Soda Process (removes both temporary and permanent hardness by precipitating calcium carbonate and magnesium hydroxide) and reverse osmosis (a physical process that removes dissolved salts, including hardness ions).

The choice of method depends on the degree of hardness, the volume of water to be treated, and the intended use of the water.