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.