Q.Answer the following questions.
A water sample, which did not give lather with soap, was found to contain Ca(HCO3)2 and Mg(HCO3)2. Which chemical will make this water give lather with soap ? Explain with the help of chemical reactions.

Step 1. Identify the cause of hardness. The water sample contains dissolved calcium and magnesium bicarbonates, Ca(HCO3)2 and Mg(HCO3)2; the Ca²⁺ and Mg²⁺ ions in solution are what react with soap to form an insoluble scum instead of a lather, i.e. they cause (temporary) hardness.

Step 2. Choose the softening agent. Sodium carbonate is used as a water softener because it precipitates these dissolved calcium and magnesium salts out of solution as insoluble carbonates, removing the Ca²⁺/Mg²⁺ that was causing the hardness.

Step 3. Write the reactions. Ca(HCO3)2(aq) + Na2CO3(aq) → CaCO3(s) + 2NaHCO3(aq); similarly, Mg(HCO3)2(aq) + Na2CO3(aq) → MgCO3(s) + 2NaHCO3(aq). …

Applying sodium carbonate's water-softening reaction (section 8.3.1) to both the calc …

• Giving only the calcium reaction and forgetting the parallel magnesium reaction, since the sample explicitly contains both salts. …