Civil Engineering MCQAE Level Civil Engineering
In the lime-soda process for water softening, temporary hardness (due to Ca(HCO3)2) is removed by adding:
Environmental EngineeringHardness Removal Lime SodaMEDIUM
Choose the correct answer
A
Soda ash (Na2CO3) only
B
Lime (Ca(OH)2) only
C
Both lime and soda ash
D
Sodium chloride
Correct Answer
B. Lime (Ca(OH)2) only
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Temporary hardness is removed by lime only: Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 (ppt) + 2H2O. Permanent hardness (CaSO4, CaCl2) requires soda ash: CaSO4 + Na2CO3 -> CaCO3 (ppt) + Na2SO4. Mg hardness needs excess lime.
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