Why is calcium fluoride insoluble in water? $\ce{NaCl}$ is soluble, but $\ce{CaF2}$ is not. Why is this?
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1$\begingroup$ Please have a look at the recent topics of solubility and judge and link the answers that concern also this question. $\endgroup$– cbeleites unhappy with SXMay 23, 2012 at 14:45
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1$\begingroup$ Without specifying the solvent, the meaning is unclear. I guess you meant soluble in water, but please write that out specifically. $\endgroup$– Gaurang TandonMar 15, 2018 at 11:04
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1 Answer
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This answer to a question about the solubility of silver nitrate and silver chloride will give you a good starting point to answer your question.
In order to dissolve a salt, you have to break apart the ions and hydrate them in solution. You can use the enthalpies of hydration of the ions, and the crystal lattice energy of the solid, to predict which compounds will dissolve. There is quite a nice discussion about how to think about solubilities of salts, and what factors affect solubility using a Born-Haber cycle, in this reference. The values for enthalpies of hydration and crystal lattice energies of sodium chloride and calcium fluoride can be found in standard chemistry texts or with an on-line search.
If you have more questions after looking at this material, come back and ask for clarification.
answered May 24, 2012 at 20:32
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1$\begingroup$ The calcium cation is much bigger in size compared to the fluorine anion, hence has less polarising power. The polarisation effects are way much less due to the additional small size (relative) of the fluorine anion (low polarisability). This leads me to conclude that calcium fluoride is highly ionic in nature, and should be soluble in water. Where have I gone wrong? $\endgroup$ Aug 29, 2017 at 4:23