An Undesirable Precipitation Reaction



 Boiler scale almost fills this hot-water pipe. The deposits consist mostly of CaCO3 with some MgCO3.

 Limestone (CaCO3) and dolomite (CaCO3 ? MgCO3), which are widespread on Earth’s surface, often enter the water supply. However, in the presence of dissolved carbon dioxide (from the atmosphere), calcium carbonate is converted to soluble calcium bicarbonate:

 [Ca(HCO3)2]: CaCO3(s) + CO2(aq) + H2O(l)  =  Ca21(aq) + 2HCO3-(aq)

where HCO3-is the bicarbonate ion. Water containing Ca2 and/or Mg2 ions is called hard water, and water that is mostly free of these ions is called soft water. Hard water is unsuitable for some household and industrial uses. When water containing Ca2+ and HCO3- ions is heated or boiled, the solution reaction is reversed to produce the CaCO3 precipitate


Ca2+(aq) + 2HCO3- (aq) =  CaCO3(s) + CO2(aq) + H2O(l)


and gaseous carbon dioxide is driven off: CO2(aq) + CO2(g) 

Solid calcium carbonate formed in this way is the main component of the scale that accumulates in boilers, water heaters, pipes, and teakettles. A thick layer of scale reduces heat transfer and decreases the effi ciency and durability of boilers, pipes, and appliances. In household hot-water pipes it can restrict or totally block the fl ow of water. A simple method used by plumbers to remove scale deposits is to introduce a small amount of hydrochloric acid, which reacts with (and therefore dissolves)
CaCO3:  CaCO3(s) +  2HCl(aq) = CaCl2(aq) 1 H2O(l) + CO2(g)

In this way, CaCO3  is converted to soluble CaCl2..


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