The Chemistry of Solids, Part 5 - Solution Equilibria of Slightly Soluble Ionic Compounds

Greetings,

This post explores the behavior of "insoluble" ionic solids in water solution in terms of a chemical equilibrium.  The solubility product constant, Ksp, and its relationship to compound solubility is also discussed.

Ionic compounds considered to be "insoluble" are in fact very-slightly soluble in aqueous solution.  There then exits a dynamic equilibrium between ions contained in the solid crystal matrix and hydrated ions in solution.  Two processes occur at the same time and at the same rate; namely, the release of ions from the solid into solution and the absorption of hydrated ions back into the crystal matrix.

The following drawing explains the method of obtaining the molar solubility (S) from the compound formula and solubility-product constant (Ksp).

 
 Note that the molar concentration (indicated by square brackets) of the solid compound is not included in the final solubility product expression.  This is because the solid molar concentration is constant and therefore is assigned a value of "1".  The expression obtained for molar solubility shows a simple exponential relationship between the solubility product constant and molar solubility!

That's all for now.  Have a good one!

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