Page 101 - Instant notes

C5
SOLUBILITY

Key Notes
Partially soluble salts only partly dissolve in solution. An
equilibrium between the ions and the solid salt is established and
a saturated solution of the ions is produced. The equilibrium
dissociation constant for this process is called the solubility
product, K sp . For partially soluble salts, the solubility of the salt,
s, is simply determined by K sp .

When a common ion (an ion which is part of the equilibrium
reaction) is added to the solution, the solubility of the salt
decreases. This is consistent with Le Chatelier’s principle, as the
equilibrium position changes to remove the ion from solution.
When an inert ion (which takes no part in the equilibrium
reaction) is added, the solubility of the salt increases. This is due
to the energetically favorable electrostatic interactions between
the inert ions and the salt ions, which stabilize the ions in
solution, favoring the dissociation of more salt.
Related topics Fundamentals of equilibria Thermodynamics of ions in
(C1) solution (E2)
Ions in aqueous solution
(E1)

Solubility equilibria

Partially (or sparingly) soluble salts are salts that only partly dissolve, forming a
saturated solution of ions. For these systems an equilibrium exists between the solid salt
and the dissolved ions:

The equilibrium constant for this reaction is often called the solubility product, K sp, and
is given by:

+
since MX is a pure solid (see Topic C1), where and are the activities of the M ion
−
(the cation, see Topic E1) and the X ion (the anion, see Topic E1). A good example
−
+
would be solid silver chloride, which only partially dissolves into Ag and Cl ions. By
substituting for the activity of the ions (see Topics C1 and E2):