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We know that almost all salts with sodium as cation are water-soluble. But there are a few examples where the salt is water-insoluble; such as sodium bismuthate, sodium fluorosilicate, and much more.

I think that's because water can't separate the ions in these examples. But if true, are there other factors that cause this strange solubility in some sodium salts?

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  • $\begingroup$ I guess that's kinda follow up of chemistry.stackexchange.com/questions/82328/… $\endgroup$
    – Mithoron
    Aug 18, 2023 at 22:31
  • $\begingroup$ related chemistry.stackexchange.com/questions/75989/… $\endgroup$
    – Mithoron
    Aug 18, 2023 at 22:34
  • $\begingroup$ The first question is just about mentioning some sodium salts which are water-insoluble. But the second question (Nilay's answer) has a good point. $\endgroup$ Aug 18, 2023 at 23:43
  • $\begingroup$ But how are those few anions capable to make so strong bonds with sodium that it's very hard to break? $\endgroup$ Aug 18, 2023 at 23:44
  • $\begingroup$ It's not a matter strong bonds, more like fitting into crystal lattice and low enthalpy of solvation. $\endgroup$
    – Mithoron
    Aug 18, 2023 at 23:58

1 Answer 1

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Having fluorine as the element ionically bonded to sodium is one factor that inhibits solubility. Like all other pretransition metal ions (which are hard acids), bonding to fluorine bases is typically stronger than bonding to similarly charged bases containing other elements, making the lattice energy/solvation balance less favorable.

We can see this in the binary compounds, which means halides (other ionic binary compounds of sodium react with water). Usually we do not list sodium fluoride as an "insoluble" sodium compound. But the respective Wikipedia articles show that it has seven times less solubility in water than sodium chloride (molar basis), and sodium bromide is similar to the chloride rather than the fluoride.

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