Chemistry Hard and soft water, Temporary hardness, Permanent hardness

Topics to be covered

`=>` Hard and soft water
`=>` Temporary hardness
`=>` Permanent hardness

Hard and Soft Water

`color{green}(★)` 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.

`color{green}(★)` 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.

`color{green}(★)` Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.

`color{green}(★)` Hard water forms scum/precipitate with soap. Soap containing sodium stearate `color{red}((C_(17)H_(35)COONa))` reacts with hard water to precipitate out `color{red}(Ca//Mg)` stearate.

`color{red}(2C_(17) H_(35) COONa (aq) + M^(2+) (aq) → (C_(17)H_(35)COO)_2M ↓ +2Na^(+) (aq) ; M)` is `color{red}(Ca//Mg)`

`color{green}(★)` 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.

`color{green}(★ " The hardness of water is of two types:")`

(i) temporary hardness, and (ii) permanent hardness

Temporary Hardness

`color{green}(★ "Temporary hardness is due to the presence of magnesium and calcium hydrogencarbonates. It can be removed by :")`

`color{green}("(𝐢) 𝐁𝐨𝐢𝐥𝐢𝐧𝐠:")` During boiling, the soluble `color{red}(Mg(HCO_3)_2)` is converted into insoluble `color{red}(Mg(OH)_2)` and `color{red}(Ca(HCO_3)_2)` is changed to insoluble `color{red}(CaCO_3)`. It is because of high solubility product of `color{red}(Mg(OH)_2)` as compared to that of `color{red}(MgCO_3)`, that `color{red}(Mg(OH)_2)` is precipitated. These precipitates can be removed by filtration. Filtrate thus obtained will be soft water.

`color{red}(Mg(HCO_3)_2 oversettext(Heating)→ Mg(OH)_2 ↓ +2CO_2 ↑)`

`color{red}(Ca(HCO_3)_2 oversettext(Heating)→ CaCO_3 ↓ +H_2O +CO_2 ↑)`

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

`color{red}(Ca(HCO_3)_2 +Ca(OH)_2 → 2CaCO_3 ↓ + 2H_2O)`

`color{red}(Mg(HCO_3)_2 +2Ca (OH)_2 → 2CaCO_3 ↓ + Mg (OH)_2 ↓ + 2H_2O)`

Permanent Hardness

`color{green}(★)` 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:

`color{green}("(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.

`color{red}(MCl_2 +Na_2CO_3 → MCO_3 ↓ +2NaCl \ \ \ \ \ \ \ ( M = Mg , Ca))`

`color{red}(MSO_4+Na_2CO_3 → MCO_3 ↓ + Na_2SO_4)`

`color{green}("(𝐢𝐢) 𝐂𝐚𝐥𝐠𝐨𝐧’𝐬 𝐦𝐞𝐭𝐡𝐨𝐝:")` Sodium hexametaphosphate `color{red}((Na_6P_6O_(18)))`, commercially called ‘𝐜𝐚𝐥𝐠𝐨𝐧’, when added to hard water, the following reactions take place.

`color{red}(Na_6P_6O_(18) → 2Na^(+) + Na_4 P_6O_(18)^(2-) \ \ \ \ \ ( M = Mg , Ca))`

`color{red}(M^(2+) + Na_4P_6O_(18)^(2-) → [Na_2MP_6O_(18)]^(2-) +2Na^(+))`

The complex anion keeps the `color{red}(Mg^(2+))` and `color{red}(Ca^(2+))` ions in solution.

`color{green}("(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 `color{red}((NaAlSiO_4))` can be written as `color{red}(NaZ)`. When this is added in hard water, exchange reactions take place.

`color{red}(2NaZ (s) +M^(2+) (aq) → MZ_2 (s) +2Na^(+) (aq) \ \ \ \ \ \ ( M = Mg , Ca))`

`color{green}(★)` 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.

`color{red}(MZ_2(s) +2NaCl(aq) → 2NaZ(s) +MCl_2(aq)) `

`color{green}("(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 - `color{red}(SO_3H)` group and are water insoluble. Ion exchange resin `color{red}((RSO_3H))` is changed to `color{red}(RNa)` by treating it with `color{red}(NaCl)`. The resin exchanges `color{red}(Na^+)` ions with `color{red}(Ca^(2+))` and `color{red}(Mg^(2+))` ions present in hard water to make the water soft. Here `color{red}(R)` is resin anion.

`color{red}(2RNa (s) +M^(2+) (aq) → R_2M (s) +2Na^(+)(aq))`

`color{green}(★)` The resin can be regenerated by adding aqueous `color{red}(NaCl)` solution.

`color{green}(★)` Pure de-mineralised (de-ionized) water free from all soluble mineral salts is obtained by passing water successively through a cation exchange (in the `color{red}(H^+)` form) and an anionexchange (in the `color{red}(OH^–)` form) resins:

`color{red}(2RH(s) +M^(2+) (aq) ⇌ MR_2(s) +2H^(+)(aq))`

`color{green}(★)` In this cation exchange process, `color{red}(H^+)` exchanges for `color{red}(Na^+, Ca^(2+), Mg^(2+))` and other cations present in water. This process results in proton release and thus makes the water acidic. In the anion exchange process:

`color{red}(RNH_2(s) + H_2O(l) ⇌ RNH_3^(+) . OH^(-) (s))`

`color{red}(RNH_3^(+) . OH^(-) (s) + X^(-) (aq) ⇌ RNH_3^(+) . X^(-) (s) + OH^(-)(aq))`

`color{green}(★)` `color{red}(OH^–)` exchanges for anions like `color{red}(Cl^–, HCO_3^–, SO_4^(2–))` etc. present in water. `color{red}(OH^–)` ions, thus, liberated neutralise the `color{red}(H^+)` ions set free in the cation exchange.

`color{red}(H^(+) (aq) + OH^(-) (aq) → H_2O(l))`

`color{green}(★)` The exhausted cation and anion exchange resin beds are regenerated by treatment with dilute acid and alkali solutions respectively.