Editing Solubility equilibrium (section)

== Definitions ==
A solubility equilibrium exists when a [[chemical compound]] in the solid state is in [[chemical equilibrium]] with a [[Solution (chemistry)|solution]] containing the compound. This type of equilibrium is an example of [[dynamic equilibrium]] in that some individual molecules migrate between the solid and solution phases such that the rates of [[Dissolution (chemistry)|dissolution]] and [[Precipitation (chemistry)|precipitation]] are equal to one another. When equilibrium is established and the solid has not all dissolved, the solution is said to be saturated. The [[concentration]] of the solute in a saturated solution is known as the [[solubility]]. Units of solubility may be molar (mol&nbsp;dm<sup>−3</sup>) or expressed as mass per unit volume, such as μg&nbsp;mL<sup>−1</sup>. Solubility is temperature dependent. A solution containing a higher concentration of solute than the solubility is said to be [[supersaturation|supersaturated]]. A supersaturated solution may be induced to come to equilibrium by the addition of a "seed" which may be a tiny crystal of the solute, or a tiny solid particle, which initiates precipitation.{{cn|date=April 2023}}

There are three main types of solubility equilibria.
# Simple dissolution.
# Dissolution with dissociation reaction. This is characteristic of [[salts]]. The equilibrium constant is known in this case as a solubility product.
# Dissolution with ionization reaction. This is characteristic of the dissolution of [[weak acid]]s or [[weak base]]s in aqueous media of varying [[pH]].

In each case an [[equilibrium constant]] can be specified as a quotient of [[activity (chemistry)|activities]]. This equilibrium constant is [[dimensionless]] as activity is a dimensionless quantity. However, use of activities is very inconvenient, so the equilibrium constant is usually divided by the quotient of activity coefficients, to become a quotient of concentrations. See {{section link|Equilibrium chemistry|Equilibrium constant}} for details. Moreover, the activity of a solid is, by definition, equal to 1 so it is omitted from the defining expression.

For a chemical equilibrium
<math display="block">\mathrm A_p \mathrm B_q \leftrightharpoons p\mathrm A + q\mathrm B</math>
the solubility product, ''K''<sub>sp</sub> for the compound A<sub>''p''</sub>B<sub>''q''</sub> is defined as follows
<math display="block">K_\mathrm{sp} = [\mathrm A]^p[\mathrm B]^q</math>
where [A] and [B] are the concentrations of A and B in a [[saturated solution]]. A solubility product has a similar functionality to an equilibrium constant though formally ''K''<sub>sp</sub> has the [[dimension]] of (concentration)<sup>''p''+''q''</sup>.