Question

Permanent hardness cannot be removed by

• A. Using washing soda
• B. Calgon's method
• C. Clark's method
• D. Ion exchange method

Hint:

Insoluble calcium carbonate precipitates out and no longer produces hardness.

Answer 1

The Correct Option is C

To understand why permanent hardness cannot be removed by Clark's method, we first need to define what permanent hardness is and how different methods work to remove water hardness.

Understanding Permanent Hardness 

Water hardness is primarily due to dissolved calcium and magnesium salts. Permanent hardness is specifically caused by calcium and magnesium sulfates or chlorides, which do not precipitate easily.

Methods to Remove Hardness

1. Using Washing Soda (Na₂CO₃):

This method involves adding washing soda to hard water. It reacts with the calcium and magnesium salts to form insoluble carbonates, which precipitate out:

1. \(Ca^{2+} + CO_3^{2-} \rightarrow CaCO_3 \, (precipitate)\)

This process is effective for both temporary and permanent hardness.

1. Calgon's Method:

Calgon, or sodium hexametaphosphate, works by sequestering calcium and magnesium ions, forming soluble complexes, which prevents them from reacting and forming hard water compounds:

1. \(Na_2[Na_4(PO_3)_6] + Ca^{2+} \rightarrow [CaNa_2(PO_3)_6]^{2-} + 2Na^+\)

This method is also effective for both temporary and permanent hardness removal.

1. Clark's Method:

This method involves the addition of lime (Ca(OH)₂), which is effective in removing temporary hardness caused by bicarbonates.

1. \(Ca(HCO_3)_2 + Ca(OH)_2 \rightarrow 2CaCO_3 \, (precipitate) + 2H_2O\)

However, Clark's method is not effective for permanent hardness, as it does not address sulfates or chlorides of calcium and magnesium.

1. Ion Exchange Method:

This involves exchanging calcium and magnesium ions in water with sodium or hydrogen ions using a resin. The resins effectively remove all types of hardness, both temporary and permanent.

1. \(2 resin-Na^+ + Ca^{2+} \rightarrow resin-Ca^{2+} + 2 Na^+\)

Conclusion

Clark's method is only effective for removing temporary hardness and not permanent hardness, which makes it the correct answer to the question: Permanence hardness cannot be removed by Clark's method.

Answer 2

Clark's method is used for the removal of temporary hardness of water.

In Clark's process, slaked lime, Ca(OH)_2​ is added to the hard water. Insoluble calcium carbonate precipitates out and no longer produces hardness.

• Ca(HCO₃​)₂​ (aq) + Ca(OH)₂ ​(aq) → 2CaCO₃ ​(s) + 2H₂​O (l)
• Mg(HCO_3​)₂ ​(aq) + Ca(OH)₂ ​(aq) → CaCO₃ ​(s) + MgCO₃ ​(s) + 2H₂O (l)

Slaked lime is itself a source of calcium ions and therefore of hardness, so care must be taken to avoid adding an excess of it.