(a) Give two reasons why carbon(IV) oxide is used for extinguishing fire (b) Explain briefly the water softening action of cation exchane resins.

Answer Details

(a) Why carbon(IV) oxide is used to extinguish fire

• It is denser (heavier) than air, so it settles over the burning material and forms a blanket that cuts off the supply of oxygen (air) needed for burning.
• It does not support combustion and is itself non-combustible, so once it covers the flame the fire goes out.

(b) Water-softening action of a cation-exchange resin

A cation-exchange resin carries loosely held sodium ions, \(Na^+\) (or \(H^+\)) on its surface. When hard water containing the \(Ca^{2+}\) and \(Mg^{2+}\) ions that cause hardness passes through the resin, the resin exchanges its \(Na^+\) ions for the \(Ca^{2+}\) and \(Mg^{2+}\) ions, which are held by the resin: \[2NaR + Ca^{2+} \to CaR_2 + 2Na^+\] The water leaving the resin now contains only \(Na^+\) ions, which do not cause hardness, so the water is softened. The exhausted resin is regenerated by running concentrated brine (sodium chloride solution) through it.