Solubility is defined as the limiting amount of solute that can dissolve in a solvent under a given set of physical conditions. The chemical properties of interest to us are present in aqueous solutions as ions. Certain combinations of these ions lead to compounds which have low solubility. Once the solubility capacity is exceeded the compounds precipitate from solution as solids. Therefore, precipitation of solid materials, which may form scale, will occur if:
- The water contains ions which are capable of forming compounds of limited solubility.
- There are changes in the physical conditions or water compositions which are lowering the solubility.
A solution that contains less solute than required for saturation is called an unsaturated solution.
A solution, whose concentration is higher than that of a saturated solution due to any reason, such as change in other species concentration, temperature, etc., is said to be supersaturated. When the temperature or concentration of a solvent is increased, the solubility may increase, decrease, or remain constant depending on the nature of the system. For example, if the dissolution process is exothermic, the solubility decreases with increased temperature; if endothermic, the solubility increases with temperature.