Just as a chemical reaction will reach an equilibrium, so to does the solubility of a substance. When a soluble solid is added to a solvent, the solid ionises and enters a state of equilibrium within the water. We call this equilibrium the solubility equilibria (Ksp). It is the point at which there is balance between the water ionising the solid, and the ions finding each other again and reforming into solid. Just like Keq, the Ksp can be disturbed, and following Le Chatelier's Principle, will shift in order to restore balance.
EXAMPLE 1:
Pb(NO3)2 (aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)
1. Ionise
2. Remove spectators
3. Make the solid the reactant
4. Change to a dynamic equilibrium symbol (⇋)
5. Calculate Ksp (exactly the same as Keq: P.O.R.K! Ignoring the solids and liquids)
At any point during the reaction, we can determine whether the system is at equilibrium (same as we would calculate Q with Keq). We call this ionic product.
If the ionic product = Ksp then the system is at equilibrium
If the ionic product < Ksp then the forward reaction will be favoured (more solid will ionise)
If the ionic product > Ksp then the reverse reaction will be favoured (ions will form solid/precipitate)