Hard Water and Soft Water - Practice Questions & MCQ

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Hard and Soft Water

Water is said to be soft if it produces sufficient lather with the soap and water is described as being hard if it forms an insoluble scum before it forms lather with soap. The hardness of natural water is generally caused by the presence of the bicarbonates and sulphates of calcium and magnesium but in fact all soluble salts that form a scum with soap cause hardness.

$\mathrm{Ca^{2+}(aq)\: +\: 2C_{17}H_{35}COO^{-}(aq)\: \rightarrow \: (C_{17}H_{35}COO)_{2}Ca}$

$\mathrm{Mg^{2+}(aq)\: +\: 2C_{17}H_{35}COO^{-}(aq)\: \rightarrow \: (C_{17}H_{35}COO)_{2}Mg}$

Soap will not produce lather with water until all the calcium and magnesium ions have been precipitated. Hard water thus wastes soap. The hardness of water is of two types:

Temporary hardness
Permanent hardness

Softening of Water - Removing Temporary Hardness

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

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.
$\mathrm{Mg}\left(\mathrm{HCO}_{3}\right)_{2} \: \overset{\mathrm{Heating}}{\rightarrow} \mathrm{Mg}(\mathrm{OH})_{2} \downarrow+2 \mathrm{CO}_{2} \uparrow$
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.
$\mathrm{Ca}\left(\mathrm{HCO}_{3}\right)_{2}+\mathrm{Ca}(\mathrm{OH})_{2} \rightarrow 2 \mathrm{CaCO}_{3} \downarrow+2 \mathrm{H}_{2} \mathrm{O}$

Removal of Permanent Hardness

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:

Washing soda: It removes both the temporary and permanent hardness by converting soluble calcium and magnesium compounds into insoluble carbonates.
$\mathrm{CaCl_{2}\: +\: Na_{2}CO_{3}\: \rightarrow \: CaCO_{3}\: +\: 2NaCl}$
Permutit: Permutit is the technical name given to certain hydrated silicates of aluminium and sodium. It is obtained by fusing sodium carbonate, china clay, silica or quartz. The remaining crystalline mass sodium alumino-silicate or sodium zeolite(Na₂Al₂Si₂O₈.xH₂O) s technically called as permutit. the sodium ions of permutit are exchanged with calcium and magnesium ions, removing the hardness of the water, when hard water is passed through it.
$\mathrm{Na_{2}Al_{2}Si_{2}O_{8}.xH_{2}O\: +\: Ca^{2+}\: \rightarrow \: CaAl_{2}Si_{2}O_{8}.xH_{2}O\: +\: 2Na^{+}}$
These ions can be re-exchanged by treating it with brine(NaCl) solution.
$\mathrm{CaAl_{2}Si_{2}O_{8}.xH_{2}O\: +\: 2NaCl\: \rightarrow \: Na_{2}Al_{2}Si_{2}O_{8}.xH_{2}O\: +\: CaCl_{2}}$
This method is useful for the removal of both temporary and permanent hardness of water.
Calgon: The complex salt of metaphosphoric acid, sodium hexametaphosphate(NaPO₃)₆, is known as calgon. It is represented as Na₂[Na₄(PO₃)₆]. Calcium and magnesium salts present in hard water react with calgon to give complex salts.
$\mathrm{2CaSO_{4}\: +\: Na_{2}[Na_{4}(PO_{3})_{6}]\: \rightarrow \: Na_{2}[Ca_{2}(PO_{3})_{6}]\: +\: 2Na_{2}SO_{4}}$
By ion exchange resins: Ion exhange resins are the most popular water softeners these days. These resins are synthetic substances. The cation exchanger consists of granular insoluble organic acid resin having giant molecules with -SO₃H or -COOH groups while the anion exchanger contains giant organic mmolecules with basic groups derived from amines.