Multiple choice

Temporary hardness of water is due to the presence of

  1. magnesium sulphate

  2. calcium chloride

  3. sodium bicarbonate

  4. calcium bicarbonate
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D Correct answer
Explanation

 Temporary hardness is due to the presence of calcium hydrogencarbonate Ca(HCO3)2(aq) and magnesium hydrogencarbonate Mg(HCO3)2(aq). Both calcium hydrogencarbonate and magnesium hydrogencarbonate decompose when heated. The original insoluble carbonate is reformed. This happens when water is boiled. The precipitation reactions are as follows:   

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