Solubility is commonly expressed as a concentration; for example, as g of solute per kg of solvent.The maximum equilibrium amount of solute that can dissolve per amount of solvent is the solubility of that solute in that solvent under the specified conditions. The advantage of expressing solubility in this manner is its simplicity, while the disadvantage is that it can strongly depend on the presence of other species in the solvent.
Solubility constants are used to describe saturated solutions of ionic compounds of relatively low solubility. The solubility constant is a special case of an equilibrium constant. It describes the balance between dissolved ions from the salt and undissolved salt. The solubility constant is also "applicable" to precipitation, the reverse of the dissolving reaction. As with other equilibrium constants, temperature can affect the numerical value of solubility constant. The solubility constant is not as simple as solubility, however the value of this constant is generally independent of the presence of other species in the solvent.
The Flory–Huggins solution theory is a theoretical model describing the solubility of polymers. The Hansen solubility parameters and the Hildebrand solubility parameters are empirical methods for the prediction of solubility. It is also possible to predict solubility from other physical constants such as the enthalpy of fusion.
The partition coefficient is a measure of differential solubility of a compound in a hydrophobic solvent. The logarithm of these two values enables compounds to be ranked in terms of hydrophilicity.The energy change associated with dissolving is usually given per mole of solute as the enthalpy of solution.