Hard Water

Hard Water 

Rain water is almost pure (may contain some dissolved gases from the atmosphere). Being a good solvent, when it flows on the surface of the earth, it dissolves many salts. Presence of calcium and magnesium salts in the form of hydrogencarbonate, chloride and sulphate in water makes water ’hard’. Hard water does not give lather with soap. Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.

Hard water forms scum/precipitate with soap. Soap containing sodium stearate (C₁₇H₃₅COONa) reacts with hard water to precipitate out Ca/Mg stearate.

2C₁₇H₃₅COONa (aq)  +  M²⁺ (aq)   →   (C₁₇H₃₅COO)₂M  +  2Na⁺ (aq);  M is Ca/Mg

It is,  therefore, unsuitable for laundry. It is harmful for boilers as well, because of deposition of salts in the form of scale. This reduces the efficiency of the boiler. The hardness of water is of two types: (i) Temporary hardness, and (ii) Permanent hardness. 

Temporary Hardness

Temporary hardness is due to the presence of magnesium and calcium hydrogencarbonates. It can be removed by :

(i) Boiling : During boiling, the soluble Mg(HCO₃)₂ is converted into insoluble Mg(OH)₂ and Ca(HCO₃)₂ is changed to insoluble CaCO₃. It is because of high solubility product of Mg(OH)₂ as compared to that of MgCO₃, that Mg(OH)₂ is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

(ii) Clark’s method: In this calculated amount of lime is added to hard water. It precipitates out calcium carbonate and magnesium hydroxide which can be filtered off.

Ca(HCO₃)₂  +  Ca(OH)₂   →   2CaCO₃ ↓  +  2H₂O ↑

Mg(HCO₃)₂  +  2Ca(OH)₂   →   2CaCO₃ ↓ +  Mg(OH)₂ ↓  +   2H₂O

Permanent hardness

It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water. Permanent hardness is not removed by boiling. It can be removed by the following methods.

(i) Treatment with washing soda (sodium carbonate): Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.

MCl₂  +  Na₂CO₃   →   MCO₃   ↓ +  2NaCl  (M= Mg, Ca)

MSO₄  +  Na₂CO₃   →   MCO₃ ↓ +  Na₂SO₄

(ii) Calgon’s method: Sodium hexametaphosphate (Na₆P₆O₁₈), commercially called ‘calgon’, when added to hard water, the following reactions take place.

Na₆P₆O₁₈   →   2Na⁺  +  Na₄P₆O^2-₁₈            (M=Mg, Ca)

M²⁺   +   Na₄P₆O₁₈^2-  →   [Na₂MP₆O_18]^2-   +   2Na⁺

The complex anion keeps the Mg²⁺ and Ca²⁺ ions in solution.

(iii) Ion-exchange method: This method is also called zeolite/permutit process. Hydrated sodium aluminum silicate is zeolite/permutit. For the sake of simplicity, sodium aluminium silicate (NaAlSiO₄) can be written as NaZ. When this is added in hard water, exchange reactions take place.

2NaZ (s)  +  M2+ (aq)   →   MZ₂ (s)   +  2Na⁺ (aq)          (M= Mg, Ca)

Permutit/zeolite is said to be exhausted when all the sodium in it is used up. It is regenerated for further use by treating with an aqueous sodium chloride solution.

MZ₂ (s)  +  2NaCl (aq)   -   2NaZ (s)  +  MCl₂ (aq)

(iv) Synthetic resins method: Nowadays hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process. Cation exchange resins contain large organic molecule with – SO₃H group and are water insoluble. Ion exchange resin (RSO₃H) is changed to RNa by treating it with NaCl. The resin exchanges Na⁺ ions with Ca²⁺ and Mg²⁺ ions present in hard water to make the water soft.  Here R is resin anion.

2RNa (s)  +M²⁺ (aq)   →   R₂M (s)  +  2Na⁺ (aq)

The resin can be regenerated by adding aqueous NaCl solution.pure de-mineralized (de-ionized) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange (in the H⁺ form) and an anion-exchange (in the OH^- form) resins:

2RH (s)  +M²⁺ (aq)   ⇌   MR₂ (s)  +  2H⁺ (aq)

In this cation exchange process, H+ exchanges for Na⁺, Ca²⁺, Mg²⁺ and other cations present in water. This process results in proton release and thus makes the water acidic. In the anion exchange process:

RNH₂ (s)  +  H₂O (I)  ⇌   RNH₃⁺.OH- (s)

RNH₃⁺.OH^- (s)  +  X^- (aq)   ⇌   RNH₃⁺.X^- (s)  +  OH^- (aq)

OH^- exchanges for anions LIKE Cl^-,  HCO₃^-, SO₄^2- etc., present in water. OH^-  ions, thus liberated neutralize the H⁺ ions set free in the cation exchange.

 H⁺ (aq)  +  OH- (aq)   →   H₂O (I)

The exhausted cation and anion exchange resin beds are regenerated by treatment with dilute acid and alkali solutions respectively.