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According to wikipedia Ca(HCO3)2 has the following solubility values:

16.1 g/100 mL (0 °C)
16.6 g/100 mL (20 °C)
18.4 g/100 mL (100 °C)

So I assume Ca(HCO3)2 would precipitate beyond this level. However, the wikipedia page for calcium bicarbonate states that attempts to prepare compounds such as solid calcium bicarbonate by evaporating its solution to dryness invariably yield instead the solid CaCO3:

Ca(HCO3)2(aq) → CO2(g) + H2O(l) + CaCO3(s)

which leaves me wondering if, when the solution is saturated beyond ~16-18 g/100 mL, a precipitate of Ca(HCO3)2 will exist in solution? or if it also dissociating beyond that point while still in solution?

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  • $\begingroup$ What is the reference to solubility at 100 degrees? This value could very well be the equilibrium value of dissolved calcium bicarbonate which is in equilibrium with solid calcium carbonate at boiling temperatures. Until and unless we don't know how it was measured, it may be futile to discuss this value. $\endgroup$ – M. Farooq Aug 2 at 12:58
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    $\begingroup$ Surprisingly, SciFinder is quiet about its solubility and it is only Wikipedia which quotes these values. No reference sadly! $\endgroup$ – M. Farooq Aug 2 at 13:05
  • $\begingroup$ Yes, all three values are from wikipedia. I edited the text to mention it. But as you write, wikipedia doesn't cite any source... : / $\endgroup$ – Hans Aug 3 at 1:33
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    $\begingroup$ I found one encyclopedia which quoted the same values without any reference. Most likely copied from Wikipedia without any reference. This is the state of affairs these days. There is German work on the solubility of calcium bicarbonate, however it is in sodium chloride solutions. $\endgroup$ – M. Farooq Aug 3 at 3:27
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    $\begingroup$ Left on note on Wikipedia calcium bicarbonate page to add citations. $\endgroup$ – M. Farooq Aug 3 at 3:31
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I would assume that when calcium bicarbonate dissolves in water, the bicarbonate reacts to form water and carbon dioxide gas. With the carbon dioxide gas evaporating, you're inevitably left with only calcium and carbonate ions in the solution, so you can't precipitate calcium bicarbonate out of the solution.

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The CRC Handbook does not list solubility numbers for Ca(HCO3)2 or Mg(HCO3)2. We know that temporary hard water has Mg and/or Ca salts of carbonic acid that will precipitate in hot water heaters at ~60 C, which appears to be due to forced evolution of CO2 rather than an inverse solubility curve.

The solubility of CO2 (0.058 g/100mL @ 60 C) would appear to be insufficient to dissolve ~10 g of CaCO3, so I question the solubility number of 18 g @ 100C.

The likely reason for being unable to isolate solid Ca(HCO3)2 is that HCO3- ion is in a labile equilibrium with CO2(aq) and OH-(aq), or 2 HCO3- --> H2CO3 + CO3--. At higher temperatures, the CO2 simply escapes by evaporation rather than because of equilibrium processes. Stalactites form at cold temperatures because the Ca(HCO3)2 solution is exposed to a large surface, where H2O and CO2 evaporate, leaving CaCO3 behind.

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