Statement (I): Re -carbonation of water softened by lime-soda pro

Statement (I): Re -carbonation of water softened by lime-soda process results in increased hardness of the water.

Statement (II): Suspended solids, like CaSO₄ and MgSO₄, which have not settled in the sedimentation tank, get dissolved due to passage of CO₂

A. Both Statement (I) and Statement (II) are individually true and Statement (II) is the correct explanation of Statement (I)

B. Both Statement (I) and Statement (II) are individually true but Statement (II) is not the correct explanation of Statement (I)

C. Statement (I) is true but Statement (II) is false

D. Statement (I) is false but Statement (II) is true (II) is true:

Please scroll down to see the correct answer and solution guide.

Right Answer is: A

SOLUTION

Hardness in water is due to presence of carbonate and bicarbonate of multivalent cations like (Ca+2, Mg+2).

After adding lime and soda ash, treated water will generally have a pH greater than 10.

Therefore, Re-carbonation is used to reduce pH in order to stabilize the water and prevent deposition of carbonate on distribution piping.

This procedure adds carbon dioxide to water after softening. Generally, carbon dioxide is added to reduce the pH of the water to less than 8.7.

CaSO₄ + H₂O → Ca⁺²+ OH^- and

MgSO₄ + H₂O → Mg⁺²+ OH^-

These ions get dissolved in CO₂ and leads to formation of CaCO₃ and MgCO₃ which increases the hardness of water. The chemical reactions involved are:

Ca⁺²+ OH^- + CO₂ → CaCO₃ + H₂O

Mg⁺²+ OH^- + CO₂ → MgCO₃ + H₂O