Solubility is the ability of a solid, liquid, or gaseous chemical substance to dissolve in another substance (solute). The solubility of a substance depends on several factors, including the physical and chemical properties of the solute and solvent, temperature, pressure, and the pH of the solution.
The solubility of a substance is determined by the equilibrium between the soluble and insoluble components of the salt. Insoluble salts will precipitate from solution when their concentration in the solution exceeds a threshold value called the solubility limit. This threshold value can be estimated by calculating the solubility product equilibria.
Soluble salts, such as silver chloride, will dissociate into the ions Ag+ and Cu2+ in solution. When a saturated solution of AgCl is mixed with a pure source of Cu2+, such as AgNO3 or copper(II) chloride, the Cu2+ ions will be separated from the Ag+ ions by forming the insoluble salt, copper(II) sulfate. This process is also known as the law of mass action.
Nickel(II) ions form a variety of complex ions in aqueous solution, and most of these ions are insoluble in basic solutions with excess sulfur dioxide (S2). However, nickel(II) sulfide is soluble in dilute aqueous cyanide because it forms the insoluble complex ion, tetracyano nickel (Ni(CN)4). It is also soluble in aqua regia, but nickel(II) sulfide does not precipitate from this acid. This property makes it useful in the electroplating of nickel-based cathode materials.