Solubility rules

Solubilities of ionic compounds in water depend in large measure upon the difference in the strength of the ion-water attractive forces and the cation-anion attractive forces. Because these differences are usually small, they do not vary in a regular way. Thus we use solubility rules based on experimentally-determined solubilities.

Soluble ionic compounds

Ammonium (NH₄⁺) and Group I salts
The most common Group I cations are Na⁺ and K⁺.
Nitrate (NO₃^–) and ethanoate (CH₃COO^–) salts
Sulfate (SO₄^2–) salts except CaSO₄, BaSO₄, PbSO₄, SrSO₄
Halide salts (Cl^–, Br^– and I^–) except if the cation is Ag⁺ or Pb²⁺

A general observation is that ionic compounds having anions with -1 charge tend to be soluble.

Insoluble ionic compounds

Hydroxides (OH^–) and oxides (O^2–) except if the cation is Na⁺, K⁺

Sulfides (S^2–) and carbonates (CO₃^2–) except if the cation is Na⁺, K⁺, NH₄⁺,
Phosphates (PO₄^3–) except if the cation is NH₄⁺, Na⁺, K⁺

A general observation is the ionic compounds that have anions with -2 or -3 charges are insoluble.