example for a dissolved solid (left)
formation of crystals in a 4.2 m ammonium sulfate
solution. the solution was initially prepared at 20 °c and then stored for 2 days at 4 °c.
solubility is the property of a solid, liquid or gaseous chemical substance called solute to dissolve in a solid, liquid or gaseous solvent. the solubility of a substance fundamentally depends on the physical and chemical properties of the solute and solvent as well as on temperature, pressure and presence of other chemicals (including changes to the ph) of the solution. the extent of the solubility of a substance in a specific solvent is measured as the saturation concentration, where adding more solute does not increase the concentration of the solution and begins to precipitate the excess amount of solute.
insolubility is the inability to dissolve in a solid, liquid or gaseous solvent.
most often, the solvent is a liquid, which can be a pure substance or a mixture. one may also speak of solid solution, but rarely of solution in a gas (see vapor–liquid equilibrium instead).
under certain conditions, the equilibrium solubility can be exceeded to give a so-called supersaturated solution, which is metastable. metastability of crystals can also lead to apparent differences in the amount of a chemical that dissolves depending on its crystalline form or particle size. a supersaturated solution generally crystallises when 'seed' crystals are introduced and rapid equilibration occurs. phenylsalicylate is one such simple observable substance when fully melted and then cooled below its fusion point.
solubility is not to be confused with the ability to dissolve a substance, because the solution might also occur because of a chemical reaction. for example, zinc dissolves (with effervescence) in hydrochloric acid as a result of a chemical reaction releasing hydrogen gas in a displacement reaction. the zinc ions are soluble in the acid.
the solubility of a substance is an entirely different property from the rate of solution, which is how fast it dissolves. the smaller a particle is, the faster it dissolves although there are many factors to add to this generalization.
crucially, solubility applies to all areas of chemistry, geochemistry, inorganic, physical, organic and biochemistry. in all cases it will depend on the physical conditions (temperature, pressure and concentration) and the enthalpy and entropy directly relating to the solvents and solutes concerned.
by far the most common solvent in chemistry is water which is a solvent for most ionic compounds as well as a wide range of organic substances. this is a crucial factor in acidity and alkalinity and much environmental and geochemical work.