- Force of attraction between H2O molecules and the ions of the solid
This force tends to bring ions into solution. If this is the predominant factor, then the compound may be highly soluble in water.
- Force of attraction between oppositely charged ions
This force tends to keep the ions in the solid state. When it is a major factor, then water solubility may be very low.
However, it is not easy to estimate the relative magnitudes of these two forces or to quantitatively predict water solubilities of electrolytes. Therefore, it is easier to refer to a set of generalizations, sometimes called 'solubility rules', that are based upon experimentation. It's a good idea to memorize the information in this table:
NO3- - All nitrates are soluble.
Cl- - All chlorides are soluble except AgCl, Hg2Cl2, and PbCl2.
SO42- - Most sulfates are soluble. Exceptions include BaSO4, PbSO4, and SrSO4.
CO32- - All carbonates are insoluble except NH4+ and those of the Group 1 elements.
OH- - All hydroxides are insoluble except those of the Group 1 elements, Ba(OH)2, and Sr(OH)2. Ca(OH)2 is slightly soluble.
S2- - All sulfides are insoluble except those of the Group 1 and Group 2 elements and NH4+.
More About SolubilityUsing Solubility To Predict Precipitates
Solubility Product Constants