Solubility Rules

This is a list of the solubility rules for ionic solids in water. Solubility is a result of an interaction between polar water molecules and the ions which make up a crystal. Two forces determine the extent to which solution will occur:

1. 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.

2. 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:

Solubility Rules

NO₃^- - All nitrates are soluble.

Cl^- - All chlorides are soluble except AgCl, Hg₂Cl₂, and PbCl₂.

SO₄^2- - Most sulfates are soluble. Exceptions include BaSO₄, PbSO₄, and SrSO₄.

CO₃^2- - 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.

S^2- - All sulfides are insoluble except those of the Group 1 and Group 2 elements and NH₄⁺.