Concept:Total hardness is caused by two different calcium salts, so it must be removed by a reagent that precipitates calcium from both salts.
Explanation:Calcium hydrogen carbonate causes temporary hardness, while calcium sulphate causes permanent hardness.
Boiling only decomposes calcium hydrogen carbonate, so it cannot remove the permanent hardness of calcium sulphate.
A calculated amount of calcium hydroxide removes only temporary hardness, and excess calcium hydroxide adds more hardness.
Sodium carbonate reacts with both salts to form insoluble calcium carbonate.
With calcium sulphate:
CaSO4+Na2CO3→CaCO3+Na2SO4.
With calcium hydrogen carbonate:
Ca(HCO3)2+Na2CO3→CaCO3+2NaHCO3.
Because all the calcium is precipitated from solution, both types of hardness are completely removed.
Answer:The correct option is D: adding sodium carbonate.