Q.What is meant by 'demineralised' water and how can it be obtained ?

Water that does not readily form lather with soap is called hard water, and this hardness is caused by dissolved calcium and magnesium salts. Temporary hardness is caused specifically by the bicarbonates, $Ca(HCO_3)_2$ and $Mg(HCO_3)_2$, and can be removed simply by boiling, which decomposes the soluble bicarbonate into an insoluble carbonate that precipitates out: $Ca(HCO_3)_2 \xrightarrow{\Delta} CaCO_3\downarrow + H_2O + CO_2$. Permanent hardness, caused by the chlorides and sulphates of calcium and magnesium, is not removed by boiling and requires chemical treatment, such as the Clark's process or, more efficiently, synthetic ion-exchange resins. A cation-exchange resin bearing acidic $-SO_3H$ groups exchanges its $H^+$ ions for the $Ca^{2+}$ and $Mg^{2+}$ ions in the water, while a companion anion-exchange resin exchanges $OH^-$ for the anions present; the liberated $H^+$ and $OH^-$ then combine to give pure water, so passing hard water successively through both resins removes essentially all d …