16. Ions and Equilibrium;
       Acids and Bases
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       Solubility Equilibria

This rule is too simple in real situations, since small ions also permit a closer approach by the hydrating water molecules, which favors solution. The crystal-energy factor usually dominates, so salts with small ions are usually (but not always) relatively insoluble. For similar electrostatic reasons highly charged ions such as phosphates (PO43-) and carbonates (CO32-) interact strongly with cations in the crystal and therefore are less soluble than are the singly charged nitrates and perchlorates.

Silicates, with SiO44- and larger frameworks, are notoriously insoluble, which is fortunate for the development of our planet.

Although real salts do seem to separate into the readily soluble and the barely soluble, "soluble" and "insoluble" really are terms for two extremes of an equilibrium between an intact salt and hydrated ions in solution. Silver chloride is an example of a sparingly soluble salt in equilibrium with its solid:

The usual equilibrium-constant expression would be written


As long as any solid AgCl remains in contact with the solution, it provides an infinite reservoir of more material, so the effective concentration of AgCl is unchanged.

This constant term, like the water concentration term in the denominator of the Ka equilibrium expression, might as well be lumped together with the equilibrium constant. The resulting combined constant is known as the solubility-product constant, Ksp:

This tells us that, as long as a solution of Ag+ and Cl- ions is in equilibrium with solid AgCl, the product of the two ion concentrations in solution will be constant.

If more of either ion is added from the outside, then solid AgCl will precipitate until the product of ion concentrations again equals Ksp. If one ion is partially removed by a chemical reaction, or if the solution is diluted so both concentrations decrease, then more AgCl will dissolve until the ion solubility product is reached again. Solubility-product constants, like all equilibrium constants, vary with temperature, but values at 25'C are ordinarily used.

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