Question

Answer the following question.

A water sample, which did not give lather with soap, was found to contain \[\ce{Ca(HCO3)2}\] and \[\ce{Mg(HCO3)2}\]. Which chemical will make this water give lather with soap? Explain with the help of chemical reactions.

Answer 1

1. Soap does not lather in hard water due to the presence of the soluble salts of calcium and magnesium in it. So, the given water sample is hard water.
2. Hardness of hard water can be removed by the removal of these calcium and magnesium salts.
3. Sodium carbonate is used to make hard water soft as it precipitates out the soluble calcium and magnesium salts in hard water as carbonates. Thus, it will make water give lather with soap.
   e.g. \[\ce{Ca(HCO3)2_{(aq)} + Na2CO3_{(aq)}->CaCO3_{(s)} + 2NaHCO3_{(aq)}}\]