question_answer

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.

Answer:

                                                                       (i) Upon boiling, the bicarbonates of calcium and magnesium decompose to form corresponding carbonates. They being water insoluble, get precipitated and can be removed by filtration. Thus, temporary hard water becomes soft on boiling \[Ca{{(HC{{O}_{3}})}_{2}}\xrightarrow{(Boil)}\,\underset{\text{(ppt)}}{\mathop{CaC{{O}_{3}}}}\,\,+{{H}_{2}}O+C{{O}_{2}}\] \[Mg{{(HC{{O}_{3}})}_{2}}\xrightarrow{(Boil)}\,\underset{\text{(ppt)}}{\mathop{MgC{{O}_{3}}}}\,+{{H}_{2}}O+C{{O}_{2}}\] (ii) When water is added to petrol fire in order to extinguish it, petrol being lighter floats over water. As a result, the fire flares up rather than getting extinguished. (iii) When hard water is used in boilers for quite sometime, the salts present in hard water slowly form an insoluble deposit all along inside the boiler. This is called boiler scale. As the scale is a poor conductor of heat, there is wastage of fuel. Moreover, it also reduces the life of the boiler. Therefore, hard water has to be softened before being used in boilers.