Which one of the following processes of softening water may require recarbonation?
- ALime-soda processCorrect
- BSodium-cation exchange process
- CDemineralization process
- DHydrogen-cation exchanges process
Solution
Concept
In lime-soda softening, excess lime raises the pH. CO₂ is then added (recarbonation) to lower the pH and stop scale formation.
Formula
Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O
Step-by-step solution
1. Lime-soda softening uses lime (and soda) and makes water highly alkaline (pH about 10-11). 2. Fine CaCO₃ and Mg(OH)₂ particles stay in the water and later deposit on filters and pipes. 3. CO₂ is passed through the water (recarbonation) to lower the pH and convert the remaining CaCO₃ to soluble bicarbonate. 4. So recarbonation is needed in the lime-soda process.
Explanation
In the lime-soda process, extra lime is added, so the softened water has a high pH. This water can form scale in filters and pipes. Recarbonation with CO₂ brings the pH down and makes the water stable.
Why the other options are wrong
B: Sodium-cation exchange does not raise the pH, so no recarbonation is needed. C: Demineralization removes almost all ions and does not leave excess lime. D: Hydrogen-cation exchange makes the water acidic. It needs neutralization or degasification, not recarbonation.
Shortcut
Recarbonation is linked with excess lime, so think of the lime-soda process.
Common mistake
Students sometimes link recarbonation with ion exchange processes. Ion exchange does not leave water at a high pH.
Correct answer: A) Lime-soda process