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We have heard many times that this XYZ solute is soluble in this ABC solvent. But what does being soluble in mean in terms of bonding? What makes a compound soluble in some solvent and not in some other. In my text it is written that:

Lithium chloride is considered to be an ionic compound, but it is soluble in alcohol, which suggests that it also possesses a small amount of covalent character.

What does covalent character have to do with solubility? On what factors does solubility depend? Please clear this confusion.

Edit:

Inter-ionic interaction is strong and solvent has to have strong interaction with ions to dissolve them, and that means it has to be very polar solvent.

This sentence helps the most!

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    $\begingroup$ Well, if something is soluble in ethanol one could suspect it's somewhat similar to this solvent (like in the like), which is covalent, isn't it. It's pretty lame reasoning, but apparently the one used there. $\endgroup$
    – Mithoron
    Commented Aug 30, 2021 at 17:00
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    $\begingroup$ Solubility is one of the numerous parameters that have not yet been understood in chemistry. There are plenty of empirical rules that work perfectly for a group of substances. But not for all substances. There are too many exceptions. Just consider this example ! In principle, $\ce{F, Cl, Br}$ and $\ce{I}$ should have similar properties. Matter of fact, $\ce{CaCl2, CaBr2, CaI2}$ are similar. They are all extremely soluble in water. Unfortunately $\ce{CaF2}$ is absolutely insoluble in water. It is the main mineral source of fluor in the world. Why is it insoluble ? Nobody knows. $\endgroup$
    – Maurice
    Commented Aug 30, 2021 at 20:25
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    $\begingroup$ @Maurice, Thanks for emphasizing this point "Solubility is one of the numerous parameters that have not yet been understood in chemistry." Students should understand that. $\endgroup$
    – AChem
    Commented Aug 30, 2021 at 21:42
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    $\begingroup$ @MFarooq I would change that to "easy to understand" or "fully understood in all cases". $\endgroup$
    – Buck Thorn
    Commented Aug 31, 2021 at 8:13

2 Answers 2

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The key for solubility are comparable intermolecular forces solute-solute, solute-solvent, solvent-solvent.

Inter-ionic interaction is strong and solvent has to have strong interaction with ions to dissolve them, and that means it has to be very polar solvent.

If there is covalent character(true covalent molecules, or tight ion pairs), intermolecular solute-solute forces are much weaker. If solvent-solvent forces in polar solvents are much stronger, replacing them by weak forces solvent-solute is not favourable. OTOH non-polar solvent have weak mutual forces, so there is not energy problem and dissolving is generally preferred by mixing entropy raise.

Therefore, ionic salts are generally much less soluble in ethanol, compared to water. Covalent character brings compounds closer to ethanol in involved forces and solubility increases.

---- Response to comments

The strength of intermolecular forces solute-solvent is usually somewhere in the middle between solute-solute and solvent-solvent, so forces have to be evaluated in their mutual relation. If forces solute-solvent are strong, but solvent-solvent forces are stronger, solubility can be lower than for case all are rather weak.

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    $\begingroup$ So basically, can I conclude ( up to high school level) that the more a solute forms bonds or has greater attraction force with a solvent, more soluble it will be? $\endgroup$ Commented Aug 31, 2021 at 6:11
  • $\begingroup$ See the answer update. $\endgroup$
    – Poutnik
    Commented Aug 31, 2021 at 7:26
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lithium chloride is considered to be an ionic compound, but it is soluble in alcohol, which suggests that it also possesses a small amount of covalent character.

This is a somewhat elaborate case, where the underlying point is that what solubilizes in alcohol is not the same as what solubilizes in water.

Water is a very small molecule with a very large dipole moment (intramolecular charge separation). The water molecule can easily accommodate charged species (ions), which it surrounds with its dipoles, forming stabilizing ion-dipole interactions. In addition water has a large dielectric constant, which results in screened interactions between ions that reduces their mutual attraction at longer distances. A third factor is that water has structure due to hydrogen bonding, which can change in character when a solute is incorporated, with associated changes in entropy and enthalpy. When the solvent becomes more structured, as happens when a highly charged ion is dissolved, the entropy of surrounding water is lowered, which discourages solubilization. However, as mentioned before, there are strong ion-water interactions which result in a large magnitude of the enthalpy of solvation (which is negative). Importantly, the devil is in the details of the relative magnitudes of the entropy and enthalpy changes.

Alcohols have neither as large a dipole moment nor dielectric constant (they are also not as structured by hydrogen bond networks, but that counts as a plus when it comes to solubilizing power). The net result is that ions are generally less soluble in alcohols. The strong mutual attraction of oppositely charged ions is not easily overcome by attractive interactions with the solvent. The result is a very low solubility of the ions.

But there is more (the catch). $\ce{LiCl}$ can solubilize as dimers composed of one $\ce{Li+}$ and one $\ce{Cl-}$ ion. The author interprets this as suggesting that there must be a covalent interaction that holds these atoms together (although why ionic interactions are not enough is not explained). The most important point here is that the energetics of dissolution of a covalent solid (for instance sugar) versus an ionic solid (table salt) are very different, because the strength of solute-solvent interactions are very different, as are solute-solute (or ion-ion) interactions. Ions interact much more strongly with the solvent and with each other, and in order to solubilize an ionic solid you need strong ion-solvent interactions that can compensate for the strong ion-ion interactions, if ion-ion interactions are indeed broken! In the case of $\ce{LiCl}$ breaking those interactions is not entirely necessary. In a covalent solid with small solutes, solute-solute interactions are weaker and solute-solvent interactions do not need to be as strong to compensate for disrupting the solid lattice.

Overall this is not entirely satisfying. If all this can seem confusing, I agree, there are a lot of details to unravel and some of the terminology sounds very similar (solvation versus dissolution, for instance). In any case, it is important to keep in mind that what happens in the solvent is only half of the picture. The other half is how the ions are organized and interact in the solid lattice. When dissolution of an ionic solid is discussed, it is often in terms of two contributions: the lattice enthalpy (which describes how strongly ions interact in the lattice, and the enthalpy of solvation (which describes how strongly the solvent interacts with the ions)*. If you ignore how strongly the ions attract in the lattice you will not fully understand why an ionic solid solubilizes.

*entropic factors can also be important as not all solutes dissolve exothermically (driven by an enthalpy change).

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