Solubility rules let you predict, without experiment, whether an ionic compound dissolves in water or drops out as a precipitate. This reference lists the standard rules by ion class and includes a predictor that combines a cation and anion and tells you the expected outcome.
How it works
The rules sort common ions into “usually soluble” and “usually insoluble” groups, each with a short exception list. Apply them in order of reliability: the strongest rules — all Group 1, ammonium, and nitrate salts are soluble — override weaker ones. To judge any salt, find the rule for its anion, then check whether its cation appears in that rule’s exceptions. To predict a reaction, swap the ion partners and test each product; an insoluble product is the precipitate.
The hierarchy of rules
Memorise the soluble-always set first: nitrates, acetates, chlorates, perchlorates, all Group 1 cations, and ammonium. Next, the halides (chloride, bromide, iodide) and sulfates are soluble except with a handful of heavy cations such as silver, lead, and barium. Finally, carbonates, phosphates, hydroxides, and sulfides are generally insoluble except when paired with the soluble-always cations.
Example and notes
Mixing barium chloride with sodium sulfate: the new pairings are barium sulfate and sodium chloride. Sulfates are soluble except with barium, so barium sulfate is insoluble and precipitates as a white solid, while sodium chloride stays dissolved. These rules describe aqueous behaviour near 25°C; solubility rises with temperature and the rules do not apply to non-aqueous solvents, so consult a quantitative solubility table when exact amounts matter.