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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 May 12 '18 at 8:18
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    $\begingroup$ Who said greater ionic character means greater solubility? $\endgroup$ – Ivan Neretin May 12 '18 at 10:53
  • $\begingroup$ there should be balance $\endgroup$ – amish dua May 12 '18 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!) $\endgroup$ – William R. Ebenezer Feb 3 '20 at 15:29
  • $\begingroup$ From my experience by asking new questions it will be immediately marked duplicate without second thought by community members , so I decided to edit this question to make it answerable. $\endgroup$ – Chemist Feb 4 '20 at 12:27
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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 Feb 3 '20 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 Feb 3 '20 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♦ Oct 29 '20 at 16:24

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