SOLUBILITY RULES

Most of the precipitation reactions that we will deal with involve aqueous salt solutions. Remember salts are compounds which consist of metal cations like Na⁺, Ca²⁺, Cu²⁺ (or the one nonmetal molecular ion that we have discussed, ammonium - NH₄⁺) ionically bonded to nonmetal anions such as Cl^-, (including molecular anions such as hydroxide - OH^-, sulfate - SO₄^2-, phosphate - PO₄^3-, nitrate - NO₃^-, and carbonate - CO₃^2-), dissolved in water. Salts can be divided into two types: those soluble in water, and those insoluble in water. You should know some simple solubiity rules which will allow you to know which salts are soluble in water.

McMurry and Fay give two basic solubility rules:

1. A compound is probably soluble if it contains one of the following cations:

• Group 1A cation:  Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺
• Ammonium ion:  NH₄⁺

2. A compound is probably soluble if it contains one of the following anions:

• Halide:  Cl^-, Br^-, I ^- (Except:  Ag^+, Hg²⁺, Pb²⁺)
• Nitrate (NO₃^-) , perchlorate (ClO₄^-), acetate (CH₃CO₂^- ), sulfate (SO₄^2-)  (Except:  Ba²⁺, Hg₂²⁺, Pb²⁺ sulfates)

Here is a more extensive set of solubility rules:

• salts containing Group 1 metals (Li, Na, K, Cs, Rb, all with a 1+ charge) and NH₄⁺ are soluble
• nitrate NO₃^- salts are soluble
• most Cl^-, Br^-, and I ^-salts are soluble, with the noteable exceptions of salts that contain Ag⁺ and Pb²⁺ and Hg₂²⁺.
• all acetates, chlorates, and perchlorates are soluble
• most sulfate SO₄^2- are soluble with the exceptions of salts containing Ba²⁺, Pb²⁺ and Ca²⁺
• hydroxides OH^- are insoluble except the slightly soluble Ca(OH)₂, and the  soluble salts of Group 1 metals and ammonium, and Sr and Ba from Group 2
• most phosphates PO₄^3- and carbonates - CO₃^2- are  insoluble except salts of Group 1 metals and ammonium, NH₄⁺
• all oxides are insoluble except those of Group IA metals.
• most sulfides S^2- are insoluble, with the exceptions of Group I, II (slightly soluble) metals and ammonium - NH₄⁺.