MX (s) + aq →Mn+ (aq) + Xn-(aq)
Why is lithium fluoride insoluble in water?
This is caused by the fluoride ion forming strong hydrogen bonds with water molecules and by the high charge density of small Li+ ion. Both these factors cause a more ordered arrangement of the water molecules.
Compounds containing doubly charged ions are insoluble in water, because all have negative ΔS° (system). They are only soluble if the enthalpy of solution is highly exothermic.
| ΔH solution | Solubility | ΔS° (system) |
| Endothermic (+) | Soluble | Must be more positive than ΔH solution/T |
| Endothermic (+) | Insoluble | Negative or less positive than ΔH solution/T |
| Exothermic (-) | Soluble | Positive or less negative than ΔH solution/T |
| Exothermic (-) | Insoluble | Must be more negative than ΔH solution/T |
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