LiF is prominently ionic compound. I also know polar substances can be dissolved in water as "like dissolves like". Why is then LiF only weakly soluble, in comparison with LiCl, or LiBr, even though LiCl is less ionic.
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6$\begingroup$ 0.134 g/(100 mL) isn't exactly insoluble. $\endgroup$– MithoronCommented May 21 at 15:28
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$\begingroup$ @Mithoron if you google anywhere it says it is. even my textbook says that $\endgroup$– Maths loverCommented May 21 at 15:29
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1$\begingroup$ If you actually look at the Google results you will see technical references giving solubility numbers. LiF is tightly bound as a solid, so will have a lower solubility in water since it is quite happy in the solid phase. $\endgroup$– Jon CusterCommented May 21 at 15:33
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1$\begingroup$ Well, how soluble something is results from balancing the Gibbs free energies of the solid vs being in solution. You can find that the enthalpy of formation of LiF is 50% larger than LiCl, so it is not surprising that it is less soluble. $\endgroup$– Jon CusterCommented May 21 at 15:39
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1$\begingroup$ No ! There are plenty of ionic insoluble compounds, like $\ce{CaCO3}$, $\ce{BaSO4}$, $\ce{CaC2O4}$, $\ce{Ca3(PO4)2}$, etc. $\endgroup$– MauriceCommented May 21 at 15:46
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3 Answers
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Not everything ionic dissolves in water. In fact, the ability of water to dissolve ionic substances is rather limited; in most cases freely soluble salts have either cations or anions or both with only one charge. Apart from some species that form ion pairs well, like magnesium sulfate, we rarely see freely soluble salts with two or more charges on both ions.
Even when the ions have only one charge, as with lithium fluoride, that is no guarantee. Especially with fluorides, the same atomic-structure factors that favor strong ion-dipole interactions between pretransition metal ions (like lithium or magnesium) and water will promote even stronger attraction between the metal and fluoride ions, making fluoride dissolution net energy-unfavorable. Not only lithium fluoride has this property, so do calcium fluoride (which makes fluorite a common fluoride mineral rather than a dissolved species in water) and magnesium fluoride (which is sufficiently waterproof for optical coating applications).
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3$\begingroup$ And that answer also points out the difference between seemingly similar fluorides and chlorides -- MgCl2 and CaCl2 are both so soluble as to be hygroscopic, yet the fluorides are not. $\endgroup$ Commented May 21 at 16:00
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I'd like to introduce you to hard-soft acid-base theory. HSAB says that we can classify ions by whether they are hard or soft. Hard ions are small, highly charged, and not very polarizable; soft ions are large, not very highly charged, and very polarizable. Two hard ions tend to interact strongly with each other and two soft ions tend to interact strongly with each other, but one hard ion and one soft ion or one soft ion and one hard ion don't interact very much. In this case, "interact" means "want to form a precipitate and not dissolve." Li+ and F- are textbook hard ions, since even though they only have a +1/-1 charge, they are very small. So Li+ and F- like forming a precipitate together and not dissolving. As you move down the halogens, the anions get bigger and bigger and more and more polarizable; I-, for instance, is a very soft anion. That's why the lithium halides get more soluble as you go down the halides; because you're pairing a hard cation with a softer and softer anion, which makes for a not-very-favorable precipitate. To give another example, Ag+ is (in)famous for being one of the only cations to have a soluble fluoride; nearly all F- salts are insoluble, but AgF readily dissolves. That's because Ag+ is a very soft cation; it's super big, but only has a +1 charge. When paired with a hard anion like F-, it doesn't interact much, so it doesn't really want to form a precipitate.
If you're wondering why HSAB works... it's mostly based on experimental evidence. So we don't really know.
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LiF is sparingly soluble in water.
It is better to say that the solubility of LiF is less than any other halide of lithium.
In fact, the order of solubility is: $\ce{LiF < LiCl < LiBr < LiI}$
Although LiF is more ionic due to the small size of the fluoride ion, it is important to remember that the solubility of a compound doesn't always depend on how ionic or covalent is. Instead, a better parameter is thermodynamics.
The reason behind the decreased solubility of lithium fluoride can be explained as follows:
The small size of lithium as well as fluorine results in better packing, in other words, a larger amount of energy is required to break the lattice and dissolve the salt. Thus, we can say that LiF is less soluble due to the increased lattice energy, energy required to break the lattice, which in this case is greater than the hydration energy, energy released upon solvation of the ions
LiI on the other hand is quite soluble due to the fact that the size of iodine is very large in comparison to lithium.
This results in bad packing and decrease in lattice energy. In case of LiI, the lattice energy is less than the hydration energy
