Explain the following:
(i) Temporary hard water becomes soft on boiling.
(ii) Water can extinguish most fires but not petrol fire.
(iii) Hard water is softened before being used in boilers.

Explanation

(I) Upon boiling, the hydrogen carbonates of calcium and magnesium decompose to form corresponding carbonates. They, being water insoluble, get precipated and can be removed by filtration. Thus, temporary hard water becomes soft on boiling
`Ca(HCO_(3))_(2) overset("Boil")to underset("ppt")(CaCO_(3))+H_(2)O+CO_(2)`
`Mg(HCO_(3))_(2) overset("Boil")to underset("ppt")(MgCO_(3))+H_(2)O+CO_(2)`
(ii) When water is added to petrol fire in order to extinguish it, petrol lighter floats over water. As a result, the fire flares up rather than getting extinguished.
(iii) When hard water slowly form an insoluble deposit all along insdie the boiler. This is called boiler scale. As the scale is poor conductor of heat, there is wastage of fuel. Morever, it also redcues the life of the boiler. Therefore, hard water has to be softened before being used in boilers.