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I read that $\ce{MgCl2}$ and $\ce{CaCl2}$ are more soluble than $\ce{NaCl}$ in water. Solubility of $\ce{MgCl2}$ is $\pu{543 g/L}$ and that of $\ce{NaCl}$ is $\pu{360 g/L}$ (both at $20^{\circ} \pu{C}$).

I think that $\ce{NaCl}$ should be more soluble due to its higher ionic nature. $\ce{Mg^{2+}}$ and $\ce{Ca^{2+}}$ are more polarizing and have more covalent nature and thus should be less soluble.

I want to know the reason for why the opposite is happening.

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  • $\begingroup$ How about a few numbers? $\endgroup$
    – Karl
    Commented May 12, 2018 at 8:18
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    $\begingroup$ Who said greater ionic character means greater solubility? $\endgroup$ Commented May 12, 2018 at 10:53
  • $\begingroup$ there should be balance $\endgroup$
    – amish dua
    Commented May 12, 2018 at 12:57
  • $\begingroup$ @Chemist, It is not advised to add questions apart from what was originally intended by the OP. Hence the reject. Your addition of solubility data was carried forward, duly credited, but again lacked the markup. Please make complete edits and try not to add to old questions (you can ask new ones!) $\endgroup$ Commented Feb 3, 2020 at 15:29
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    $\begingroup$ @Chemist Have faith in the SE system :). Even if closed in a hurry, posts can be reopened or the closure discussed on meta. Justice will be done. $\endgroup$ Commented Feb 4, 2020 at 17:01

3 Answers 3

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There are no answers to such why questions, in case anyone is looking for an "answer". Although it is very tempting to rationalize every laboratory observation, there are no definite answers to questions about why certain chemical phenomena occur. While it may be tempting to come up with explanations that allow us to pass exams or understand concepts on a surface level, the truth is that our current understanding of chemistry is not advanced enough to accurately predict the solubility of compounds in water or other solvents. The same goes for melting and boiling points. Tools such as ChemDraw and SciFinder can provide estimates, but there is a significant margin of error in those solubility numbers and the results may be incorrect.

Let us see the complications, e.g.,

Take anhydrous $\ce{MgCl2}$, add it to water and dissolve, the reaction is exothermic, and solution can be shown to be slightly basic.

Take the hydrated version, $\ce{MgCl2.6H2O}$, and add it to water, the reaction is not exothermic in water, and water's pH does not change. So may even ask, which magnesium chloride is being discussed?

As a result, many chemists rely on macroscopic measurements such as thermodynamic properties but such properties are quiet about microscopic details and "whies". A paper titled The solubilities of some inorganic halides, Trans. Faraday Soc., 1958,54, 34-39, is relevant the OP's question with actual experimental data. The abstract reads,

"enter image description here

They have the thermodynamic data of Group I and Group II halides in Table 1.

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  • $\begingroup$ "Take anhydrous MgCl2... and solution can be shown to be slightly basic." Not really. Should be slightly acidic, unless the dehydration process left the magnesium chloride impure (e.g. by forming a basic salt). $\endgroup$ Commented Jan 23, 2023 at 15:10
  • $\begingroup$ There is a demo on this topic by the Royal Society of Chemistry. I trust their science. On Youtube, search "Reaction water NaCl, MgCl2". I will not discredit this until and unless there is a solid evidence otherwise. $\endgroup$
    – ACR
    Commented Jan 23, 2023 at 20:36
  • $\begingroup$ so, in fact, it does react to produce an imputity. Looking carefully at the video you see there is the basic salt. $\endgroup$ Commented Jan 23, 2023 at 20:52
  • $\begingroup$ Right, the basic salt is formed after hydrolysis. The starting material is not a basic salt of magnesium chloride. $\endgroup$
    – ACR
    Commented Jan 23, 2023 at 21:55
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I don't think there is any simple answer.There are some rules of thumb, all of which have exceptions. Most chlorides are soluble. That includes both MgCl2 and CaCl2.

Solubility is the result of the balance between competing interactions: between the ions in the salt and between those ions and the water.

Many factors contribute to the solubility of a compound in water. In this case, the main difference between the compounds is the bonds’ strength.

The longer answer is to do with something called the "enthalpy of solvation".'

you can think it this way that the lattice enthalpy in MgCl2 and CaCl2 will be less and can be easily be overpowered by hydration enthalpy

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  • $\begingroup$ Why is lattice enthalpy of MgCl2 is less than NaCl $\endgroup$
    – Chemist
    Commented Feb 3, 2020 at 12:53
  • $\begingroup$ I am afraid solubility is one of the properties that cannot be properly explained yet. For example why is CaF2 absolutely insoluble ? By comparaison CaCl2, CaBr2 and CaI2 are all extremely soluble in water, as they can be dissolved in less than their weight of water. $\endgroup$
    – Maurice
    Commented Feb 3, 2020 at 14:17
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$\ce{MgCl2}$ and $\ce{CaCl2}$ are more soluble than $\ce{NaCl}$ because In $\ce{MgCl2}$ and $\ce{CaCl2}$ the ionic charge is +2 hence it can interact with more number of $\ce{H2O}$ molecules, as the number of interactions increases more amount of energy is liberated which is a hydration energy which is sufficient enough to overcome the lattice energy of the compound. Even solubility increases with increase in charge on ion.

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    $\begingroup$ If only chemistry were so simple… A brief look at the solubility chart would show this is not really how solubility is predicted/explained. $\endgroup$
    – andselisk
    Commented Oct 29, 2020 at 16:24

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