The solubility of a substance is a measure of the maximum amount that dissolves in a given volume of solution at a certain temperature. The solubility of ionic solids varies widely, depending on the temperature, the nature of the solid, and which other species are present iin the solution.
Ionic solids are strong electrolytes, and to the extent that they dissolve, they give rise to ions in solution as shown for CaF2 below. Solutions that are in equilibrium with undissolved solid are said to be saturated.
The equilibrium constant for the dissolving reaction is known as Ks.
The solubility constant Ks for an ionic solid is constant for all aqueous solutions of that solid at the same temperature.
Ionic solids are strong electrolytes, and to the extent that they dissolve, they give rise to ions in solution as shown for CaF2 below. Solutions that are in equilibrium with undissolved solid are said to be saturated.
CaF2(s) 
Ca2+(aq) + 2F–(aq)
For slightly soluble ionic solids (such as CaF2) the reaction is reactant-favoured. Ca2+(aq) + 2F–(aq)Ks(CaF2) = [Ca2+][F–]2
The equilibrium constant for the dissolving reaction is known as Ks.
The form of Ks is the same as for other equilibrium constants with the concentration of dissolved species raised to their coefficient in the balanced equation for the reaction. Note that the undissolved solid does not appear in Ks.
The solubility constant Ks for an ionic solid is constant for all aqueous solutions of that solid at the same temperature.
Ks(CaF2) = [Ca2+][F–]2
If the concentration of one of the ions in an aqueous solution is higher due to being present before dissolving occurs, in a saturated solution of the solid in that solution, the concentration of the second ion that arises exclusively from dissolving will be lower. This is necessary for the product of ion concentrations above to equal Ks.
This means that the dissolving reaction occurs to a lesser extent than in water and the solubility in a solution having a common ion is lower. This is referred to as the common ion effect.
This means that the dissolving reaction occurs to a lesser extent than in water and the solubility in a solution having a common ion is lower. This is referred to as the common ion effect.