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Question

The Permanent hardness of water is due to compound A. One of the method to remove permanent hardness of water is B whereas the temporary hardness of water is due to compound X. Boiling this compound converts X to compound Y. A, B, X, and Y, respectively can be:

A
MCl2, by boiling method, Ca(HCO3)2 and Ca(OH)2
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B
MCl2, Calgon's method, Mg(HCO3)2 and Mg(OH)2
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C
Ca(HCO3)2, ion- exchange method, Mg(HCO3)2 and MgCO3
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D
MSO4, Clark's method, Ca(HCO3)2 and CaO
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Solution

The correct option is B MCl2, Calgon's method, Mg(HCO3)2 and Mg(OH)2
Permanent hardness of water is due to soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. One of the method to remove permanent hardness of water is Calgon's method. In this method, sodium hexameta phosphate also known as calgon is added to hard water, following reaction takes place:
N6P6O182Na++Na4P6O218M2++Na4P6O218[Na2MP6O18]2+2Na+
Where, M=Mg, Ca
The complex anion keeps the Mg2+ and Ca2+ ions in the solution.
Temporary hardness of water is due to Mg(HCO3)2 and Ca(HCO3)2. During boiling, the soluble Mg(HCO3)2 is converted to insoluble Mg(OH)2 and Ca(HCO3)2 is converted to insoluble CaCO3. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.
Mg(HCO3)2heating−−−Mg(OH)2+2CO2
Ca(HCO3)2heating−−−CaCO3+H2O+2CO2

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