Question

Are all ionic compounds soluble in water?

Answer 1

Not all ionic compounds are easily soluble in water. Just because a compound is ionic doesn't mean it is automatically soluble in water.

A soluble compound defined by Sigma Aldrich is approximately `>"3.33 g/100 mL"`. Anything less is considered sparingly soluble, slightly soluble, very slightly soluble, or practically insoluble.

Examples of ionic compounds that are either borderline soluble, poorly soluble, or essentially not soluble in the least, are:

• `"Ba"("OH")_2` (`"3.89 g/100 mL"` @ `20^@ "C"`; `K_(sp) ~~ 5.0xx10^(-3)`)
• `"CaSO"_4` (`~~` `"0.2 g/100 mL"` @ `20^@ "C"`; `K_(sp) ~~ 4.93xx10^(-5)`)
• `"AgCl"` (`~~` `"0.00052 g/100 mL"` @ `20^@ "C"`; `K_(sp) ~~ 1.77xx10^(-10)`)
• `"AgI"` (`~~` `"0.0000003 g/100 mL"` @ `20^@ "C"`; `K_(sp) ~~ 8.52xx10^(-17)`)
• `"PbS"` (...Let's just call it zero. `K_(sp) ~~ 9.04xx10^(-29)`)

And for each of the above, there contains a cation (`"Ba"^(2+)`, `"Ca"^(2+)`, `"Ag"^(+)`, `"Pb"^(2+)`), and an anion (`"OH"^(-)`, `"SO"_4^(2-)`, `"Cl"^(-)`, `"I"^(-)`, `"S"^(2-)`), hence each are ionic compounds.

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If an ionic compound does happen to be easily soluble in water, it must be that...

the ion-ion interactions between the cation and the anion are weaker than the ion-dipole interactions between each ion and water molecules.

When a solid dissolves successfully in water, it must break apart its ion-ion interactions, and form new ion-dipole interactions.

Basically, the idea is that the stronger interaction can more favorably form.

• If the interaction between the cation and the anion is weak, but the interaction between each ion and water is strong, the solid is favorably soluble.
• If the interaction between the cation and the anion is strong, but the interaction between each ion and water is weak, the solid is unfavorably soluble.