Q.What do you understand by hardness of water?

Both statements are individually true, and the reason correctly explains the assertion: washing soda removes permanent hardness precisely because it converts soluble Ca2+/Mg2+ salts into insoluble carbonates that precipitate out.

Permanent hardness of water is caused by dissolved chlorides and sulphates of calcium and magnesium (unlike temporary hardness, which is caused by their bicarbonates and can be removed simply by boiling).

Washing soda, Na2CO3, removes this permanent hardness by a double-displacement (precipitation) reaction:

CaCl2(aq) + Na2CO3(aq) -> CaCO3(s) (down arrow) + 2NaCl(aq)

MgSO4(aq) + Na2CO3(aq) -> MgCO3(s) (down arrow) + Na2SO4(aq)

The soluble calcium/magnesium chloride and sulphate salts are converted into insoluble calcium/magnesium carbonate, which precipitates out and is removed (e.g. by filtration/settling), leaving the water softened.

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Permutit is hydrated sodium aluminium silicate, Na2Al2Si2O8·xH2O, used as an ion-exchange resin to soften hard water.

Hard water contains dissolved Ca2+ and Mg2+ salts. Permutit is a synthetic zeolite (a sodium aluminosilicate) that removes this hardness by ion exchange: when hard water is passed through a bed of permutit, the Na+ ions in the permutit are exchanged for the Ca2+/Mg2+ ions in the water, which get trapped in the permutit lattice while soft (Na+-ri …