Problem 25 - A Complete Resource Book in Chemistry for JEE Main

Question

By which of the following process permanent hardness of water can be removed? (a) washing soda (b) soda lime (c) sodium chloride (d) sodium bicarbonate

Step-by-Step Solution

Answer

Permanent hardness of water can be removed using washing soda (option a).

Step 1: Understanding Hardness of Water

Water hardness is due to the presence of calcium and magnesium ions. Permanent hardness is specifically caused by calcium and magnesium sulfates, which cannot be removed by boiling.

Step 2: Identifying Methods of Softening Water

To remove permanent hardness, water needs chemical treatment. Using substances that can precipitate calcium and magnesium is effective. Common methods include adding compounds that react with ions to form precipitates.

Step 3: Analyzing the Options

Examine each option: - **Washing soda (sodium carbonate):** This can remove permanent hardness by reacting with calcium and magnesium ions to form insoluble carbonates. - **Soda lime:** Primarily used to absorb CO2 and not effective for water softening. - **Sodium chloride:** Common table salt, ineffective for removing permanent hardness. - **Sodium bicarbonate:** Has limited effect on calcium and magnesium ions and is mainly used for temporary hardness.

Step 4: Conclusion

Based on the analysis, washing soda (sodium carbonate) is suitable for removing permanent hardness. When added to hard water, it causes calcium and magnesium to precipitate as carbonates, effectively softening the water.

Key Concepts

Permanent Hardness

Water hardness is an issue that many households face and is primarily caused by the presence of dissolved minerals in water. Permanent hardness is a type of water hardness specifically due to the presence of calcium and magnesium ions that form compounds such as calcium sulfate (CaSO₄) and magnesium sulfate (MgSO₄).
Unlike temporary hardness, which can be removed by boiling, permanent hardness cannot be addressed through simple heating.

• Permanent hardness remains even after heating water.
• It requires specific treatment methods to remove.

Understanding the nature of permanent hardness is essential for choosing the right approach to treat hard water and prevent issues such as scaling in pipes and reduced efficiency of soaps and detergents.

Water Softening Methods

Since permanent hardness cannot be removed by boiling, it requires chemical treatment. Various water softening methods are available to address this issue, each with its unique approach to tackling calcium and magnesium ions.

• Ion Exchange: This process involves replacing calcium and magnesium ions with sodium ions through a resin medium. While effective, it requires refilling of sodium ions periodically.
• Chemical Addition: Adding compounds like washing soda (sodium carbonate) can precipitate calcium and magnesium ions out of the water as insoluble carbonates. This method is efficient for removing permanent hardness.

By using these methods, the troublesome ions causing hardness can be effectively removed, resulting in softer water that is more suitable for household and industrial use.

Calcium and Magnesium Ions

Calcium and magnesium ions are pivotal players in the concept of water hardness. These ions originate from groundwater passing through mineral deposits such as limestone and chalk.
While they are essential nutrients for human health, their presence in water contributes to the hardening effect.

• Calcium ions (Ca²⁺) contribute to the formation of scale in pipes and appliances.
• Magnesium ions (Mg²⁺) enhance the binding of soap, reducing its lathering ability.

By targeting and removing these ions, it becomes easier to prevent the adverse effects associated with hard water, such as clogs and reduced cleaning efficiency.

Washing Soda

Washing soda, with its chemical name sodium carbonate (Na₂CO₃), plays a crucial role in softening hard water. When added to water containing calcium and magnesium ions, it reacts to form insoluble carbonates which precipitate out of the water.
This precipitate can then be removed, effectively reducing the hardness of the water.

• Washing soda is both effective and economical for household water softening.
• By converting harsh ions into solids, it improves soap effectiveness and reduces scaling.

Thus, washing soda is a favored choice in mitigating permanent hardness, providing a simple yet powerful solution for improving water quality.