Solubility product KS and solubility S
In table XI you can find of a large number of substances if they dissolve in water and, if yes, how much can dissolve befor saturating the solution.
For all substances, also for salts, there is a moment of saturarion.
Badly soluble salts reach that point much faster than well soluble salts.
The solubility of a salt mostly is indicated in the unit mol/liter, sometimes in mol/100 gr.
If a salt in water (partly) dissolves, than the ionic lattice (also partly) falls apart; the number of free particles increases.
Dissolving depends on the strenghth of the ionic lattice, but also of the temperature. At high temperatures the ionic lattice easier fals apart.
Badly soluble salts have ionic lattices that only dissociate in water for a (very) small part.
In that case there is a heterogenious and onesided equilibrium.
The small amount of silver carbonate that dissolves in water (in mol/l) is called the Solubility S at a certain temperature.
Att: in this example S mol silver carbonate will form: 2S mol silver ions and S ml carbonate ions.
The ions spread out homogeniously throughout the whole solution.
Because the solid and undissolved silvercarbonte is present in a heterogious way, the concentration of that solid in the equilibrium condition K may have the value 1.
The equilibrium constant K for badly soluble salts can be considered as: the ionic product in a saturated solution.
The ionic product (I.P.) in a saturated solution of a badly soluble salt is called the SOLUBILITY PRODUCT KS
Ks will be bigger at higher temperatures.
- As long as there is not dissolved substance on the bottom, the solution saturated (I.P.= Ks).
- If not, than you still can add and the solution is unsaturated (I.P.< Ks).
- In special cases you can have oversaturated solutions (I.P. > Ks).
You must be capable to calculate S from Ks and v.v.: Ks from S.