What is the reason the solubility of calcium sulfate in water is so low? Compared to other salts like potassium sulfate (110 g/L) or calcium nitrate (1470 g/L) the solubility of calcium sulfate seems to be very low (2.4 g/L).

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    $\begingroup$ Being a salt doesn't mean it has to be soluble $\endgroup$ – Micelle May 16 '20 at 5:48
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    $\begingroup$ That's a pretty good equivalent of "Why is Donald Trump so poor, even though he is American?" $\endgroup$ – Ivan Neretin May 16 '20 at 7:25

Solubility of calcium sulphate is quite high, compared to many much less soluble salts, like barium sulphate, calcium phosphate or calcium fluoride.

Most of insoluble minerals are salts.

Insoluble/limited solubility salts have as crystals lower Gibbs energy than dissolved, leading to solution being thermodynamically unfavourable.

That is related to mutual relation of energy of crystal lattice and of hydration of ions, and also to dissolution entropy change.

  • $\begingroup$ @ Poutnik. You are right. But the question should be rewritten so : How to explain why insoluble salts have as crystals a lower Gibbs energy than dissolved ? $\endgroup$ – Maurice May 16 '20 at 12:00
  • $\begingroup$ There is a similar situation with halogenides. Calcium iodide, bromide and chloride are extremely soluble in water. Surprisingly, Calcium fluoride is absolutely insoluble in water. Why ? $\endgroup$ – Maurice May 16 '20 at 12:00
  • $\begingroup$ Only if the opposite has to be explained as well, i.e.why soluble salts have lower G if dissolved. :-) $\endgroup$ – Poutnik May 16 '20 at 12:01

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