Solubility

Solubility refers to the concentration of a solute when the solution is in equilibrium with the pure solute. Under these conditions the solution is called "saturated". The solubility is defined at a certain temperature since the solubility often changes with temperature. In the case of gaseous solutes the pressure must also be defined.

The solubility of a given substance will vary for different solvents. In general a substance is more soluble in solvents that are similar in nature to the solute. That is ionic compounds are soluble in polar solvents, hydrocarbons are soluble in hydrocarbons, solutes with hydroxyl groups dissolve in alcohols, etc.

Electrolytes vary greatly in solubility. A familiarity with solubility is important for separations based on precipitation. There are some general rules based on the type of anion. This applies to to compounds of the common cations, Na⁺, K⁺, NH₄⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Al³⁺, Cr³⁺, Mn²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Ag⁺, Cd²⁺, Sn²⁺, Hg₂²⁺, Hg²⁺, and Pb²⁺. The term "soluble" means that the solubility is greater than about 1 g per 100 ml and "insoluble" means that the solubility is less than 0.1 g per 100ml. Substances with solubility between these limits are called "sparingly soluble".

All nitrates are soluble.

All acetates are soluble.

All chlorides, bromides and iodides are soluble except those of Ag⁺, Hg₂²⁺ and Pb²⁺. PbCl₂ and PbBr2 are sparingly soluble in cold water.

Most sulfates are soluble. Some exceptions are SrSO₄, BaSO₄ and PbSO₄. CaSO₄, Ag₂SO₄, and Hg₂SO₄ are sparingly soluble.

Almost all salts of Na⁺, K⁺, and NH₄⁺ are soluble.

All normal carbonates and phosphates are insoluble except those of the Group 1 elements (H⁺, Li⁺ Na⁺, K⁺, etc.) and NH₄⁺.

Hydroxides are insoluble except those of the Group 1 elements, ( Sr(OH)₂, Ba(OH)₂ and Ca(OH)₂ are slightly soluble).

Sulfides other than those of Group 1 and 2 elements and NH₄⁺ are insoluble.