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What is meant by .demineralised. water and how can it be obtained ? |
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Answer» Solution :Permanent hardness 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. `MCl_2 + Na_2CO_3 to MCO_3 darr + 2NaCl (M=Mg,Ca)` `MSO_4 + Na_2CO_3 to MCO_3 darr + Na_2SO_4` (ii) Calgon.s method: Sodium hexametaphosphate `(Na_6P_6O_18)` , commercially called .calgon., when added to hard water, the following reactions take place. `Na_6P_6O_18 to 2Na^(+)+ Na_4 P_6O_18^(2-)` `M^(2+) + Na_4P_6O_18^(2-) to [Na_2MP_6O_18]^(2-) + 2Na^(+)` (M=Mg, Ca) The complex anion keeps the `Mg^(2+)` and `Ca^(2+)` ions in solution. (iii) Ion-exchange method : This method is also called zeolite/permutit process. Hydrated sodium aluminium silicate is zeolite/permutit. For the sake of simplicity, sodium aluminium silicate `(NaAlSiO_2)` can be written as NaZ. When this is added in hard water, exchange reactions take place. `2NaZ_((s)) + M_((aq))^(2+) to MZ_(2+(aq)) to NZ_(2(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. `MZ_(2(s)) + 2NaCl_((aq)) to 2NaZ_((s)) + MCl_(2(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 It contain large organic molecule with -`SO_3H` group and are water insoluble. Ion exchange resin `(RSO_3H)` is changed to RNa by treating it with NaCl. The resin exchanges `Na^+` ions with `Ca^(2+)` and `Mg^(2+)` ions present in hard water to make the water soft. Here R is resin anion. `2RNa_((s))+ M_((aq))^(2+) to R_2M_((s)) + 2Na_((aq))^(+)` The resin can be regenerated by adding aqueous NaCl solution. Pure de-mineralised (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)) + M_((aq))^(2+) hArr MR_(2(s)) + 2H_((aq))^(+)` In this cation exchange process, H+ exchanges for `Na^(+), Ca^(2+), Mg^(2+)` and other cations present in water. This process results in proton release and thus makes the water acidic. Anion exchange process : `RNH_(2(s)) + H_2O_((l)) hArr RNH_(3)^(+) . OH_((s))^(-)` `RNH_3^(+). OH_((s))^(-) + X_((aq))^(+) hArr RNH_(3)^(+) . X_((s))^(-) + OH_((aq))^(+)` `OH^-` exchanges for anions like `Cl^(-), HCO_3^(-), SO_4^(2-)`etc. present in water. `OH^-` ions, thus, liberated neutralise the `H^+` ions set free in the cation exchange. `H_((aq))^(+) + OH_((aq))^(-) to H_2O_((l))` The exhausted cation and anion exchange resin beds are regenerated by treatment with dilute acid and alkali solutions respectively. |
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