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Why does Potassium Chloride less soluble than Sodium Chloride in water, isn't it supposed to be other way round? Since Potassium Chloride would be more ionic than Sodium Chloride?

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    $\begingroup$ So what? MgO is pretty ionic, yet insoluble at all. $\endgroup$ – Ivan Neretin Oct 6 '17 at 20:22
  • $\begingroup$ Well MgO does react, even with moisture from air (moderately strong base). The hydroxide has limited solubility but still greater than most other metal hydroxides. $\endgroup$ – Oscar Lanzi Oct 8 '17 at 18:35
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When dissolving, the ionic crystal’s lattice enthalpy must be broken while at the same time the hydration shell is generated around the cations and anions. The latter process is typically exergonic while the former is strongly endergonic.

How well a salt dissolves depends not only on the strength of solvation but also on the strength of the crystal lattice. If the lattice is strong, the solubility will be lower because there is an increased tendency to reform the crystal lattice.

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Sodium ion has smaller size than potassium ion.So sodium ion can more easily form a hydration sphere around it than potassium.So NaCl is more soluble than KCl.

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