Q. What is softening of water? Describe deionization method of water softening with diagram.

Ans. Softening of water: The process of removing the hardness causing salts from water is called softening of water. Hard water is unsuitable for industries as well as for domestic purposes. Therefore, it is necessary to remove the hardness of water before using in various industries, boilers etc. The softening of hard water can be done by:

Removal of hardness causing salts of calcium and magnesium.

Conversion of these salts into soluble sodium salts.

Combination of the above methods. The hardness causing salts can be removed mainly by three methods.

(a) Lime-soda method

(b) Zeolite method and

(c) Ion-exchange methods.

Ion Exchange process (Determination or Deionization):

The substances capable of exchange of ions with the electrolyte solution are called ion exchangers. The process of ion exchange is the property of a ion exchangers to exchange an ion in its structure with ion in solution that is brought in contact with it. Since all the cations and anions are completely removed in this process, so it is known as "complete deionization". Ion exchange are organic cross, linked, insoluble, high molecular weight, polymers with a porous structure, and the "functional structure" attached to the chains are responsible for the ion-exchange properties. The ion exchange or ion-exchanger resins may be classified as:

Cations exchangers: Resins containing acidic functional groups like (-COOH, -SO₃H etc.) are capable of exchanging their H⁺ ions with other cations which comes in their contact; and hence they are termed as cations exchangers e.g. styrene divine benzene copolymers which on sulphonation.

They can be represented as R-H⁺ where R^- represents the insoluble polymer matrix. The exchange reactions are:

2R^- - H⁺ + Mg⁺² = R₂Mg + 2H⁺

 2R^- - H⁺ + Ca⁺² = R₂Ca + 2H⁺

 Amberlite IR-120; and Do vex -50 are commercially available cations exchange resins.

Anion exchangers: Resin containing basic functional group like ammonia or quaternary ammonium or quaternary phosphonium or hydroxides or hydrochloride are capable of exchanging their anions in water and therefore are known as anion exchangers e.g. styrene benzene or ammine formaldehyde copolymers. These after treatment with dilute 2NaOH solution, become capable to exchange their OH^- anions with other anions present in water. They can be represented as R⁺ OH^- where R⁺  

2R⁺ + OH^- + SO₄^-2 = R₂SO₄ + 2OH^-

R⁺OH^- + Cl^- = RCl + OH^-

Process: The apparatus used for softening hard water by this method consists of two containers, one containing cations exchange resin and the other anions exchange resin over a bed of gravel. Hard water is first passed through the cations exchange resin. The cations present in hard water get exchanged with H⁺ ions of the resin. The water coming out of the first container thus contains free yyhjions in acidic in nature. It is now passed through the anion exchange resin. These resin exchangers the anions with OH^- ions.

R⁺ - OH^- + Cl^- = RCl + OH^-

The OH^- ions thus formed combine with free H⁺ ions to form H₂O molecules. all cations and anions that were present in hard water. It is known as deionised or demineralised water.