Softening hard water by boiling

Question

A test tube containing a calcium carbonate solution is heated and an insoluble precipitate is settle on bottom of the test tube. This should be considered 'temporary hard'. But a sample of this solution is mixed with a soap solution, it doesn't lather and scum is still formed. Shouldn't the sample react with soap as being soft water and consequently lather?

Answer 1

You decreased the volume of the sample by boiling, which disrupted the equilibrium between disolved and solid calcium carbonate. Since the volume decreased, the equilibrium shifted towards solid calcium carbonate to reestablish its previous equilibrium. Although some CaCO3 precipitated, there was likely still a considerable amount disolved in solution....which would still make the water somewhat hard.

CaCO3   <--------> Ca2+ + CO32-       k=[Ca2+][CO32-]

If all the above was more confusing than helpful....the short answer is that some calcium carbonate still remained in solution after boiling so the water was still hard and reacted as such with soap

Answer 2

No, you still have Ca++ ions in soln...and this causes the Ca-soap scum.

Depending upon how much heat was applied...

Ca++CO3-- +H2O  <---> Ca++OH-OH- + CO2

Both CaCO3 and Ca(OH)2 are partly soluble in water.