JEE Main · 2021 · 26 Aug 2021 · Shift 1 · Hydrogen

Deionised water must be free of every dissolved ion, cations and anions alike. In the synthetic resin method the water is passed first through a cation exchanger in its ${\mathrm{H}}^{+}$ form and then through an anion exchanger in its $\mathrm{O}{\mathrm{H}}^{-}$ form, so every cation is replaced by ${\mathrm{H}}^{+}$ and every anion by $\mathrm{O}{\mathrm{H}}^{-}$, and these combine to give water alone. The permutit method exchanges only cations, replacing them by sodium and leaving the anions in solution, while Clark's method and Calgon's method merely remove or sequester the hardness-causing ions. The method suited to preparing deionised water is the synthetic resin method.