Most chemistry textbooks contain a chart or table of solubility. These charts divide ionic compounds into two groups, “soluble” and “insoluble”. Some books even add a third category, “slightly soluble.” However, these solubility charts are misleading. Ionic compounds are never completely soluble or insoluble. They always dissociate into their ions to some extent when placed in a solvent.
The solubility of an ionic compound is determined by the strength of the interionic forces between the ions. If these ionic forces are stronger than the ion-dipole interactions between an ion and water molecules, the ion will remain undissolved in solution. Silver nitrate, for example, is an insoluble salt because the strong interionic forces between silver cations and nitrate anions are greater than the ion-dipole attractions between these ions and water molecules.
In addition, the solubility of a compound is determined by the concentration of the solvent. If a solvent is too concentrated, the ionic compound will not dissolve. If a solvent is too dilute, the ionic compound will dissolve, but it may not reach a saturated concentration.
Lead bromide is soluble in hydrochloric acid, nitric acid, and acetic acid, but it is less soluble in hydrogen bromide solution. A liter of dilute hydrogen bromide solution dissolves only 1.25 grams of PbBr2 at 11 degrees Celsius. This is because of the formation of complex ions. In contrast, the solubility of PbBr2 in DMSO and DMF is much larger because these solvents have large donor numbers that can compete with iodide anions and bind directly to the Pb2+ ions in haloplumbate complexes.