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The solubility (due to dissolution and/or dissociation and/or ionization) of a specific quantity of solute in a certain volume of a given solvent at temperature can be predicted by calculating the Gibbs free energy change: . The temperature and changes of enthalpy () and entropy () all affect the solubility of a solute. The Gibbs free energy change mathematically expresses the role of each of these three thermodynamic variables within reactions (including dissolution). Regardless of the type of reaction (a dissolution, a dissolution with dissociation or a dissolution with ionization), all chemical reactions follow the rule: if for a given amount of solute and solvent then the reaction occurs (the solute is soluble), although the velocity may be slow and depends on the potential barriers, but if then the reaction will not occur (the specified quantity of solute is not soluble). Generally it is easy to estimate ; however, it is very difficult to evaluate in most real situations. Dissociation of a salt, for instance, increases the number of states available to the sub-system salt, as solvation occurs, but decreases the number of states available to the sub-system water molecules. This works as a trade off: as the entropy of the salt ions increases the entropy of the water molecules is consequently decreased. When the overall entropy change is negative then lower temperatures favor the occurrence of the dissolution. By contrast, if the entropy change is positive then higher temperatures will favor dissolution. It is necessary to note that lower temperatures generally decrease the velocity of reactions, since a lower thermal energy decreases the probability of reactants overcoming the energy potential barriers of the intermediate products. Remember that entropy is proportional to the logarithm of the number of states available to the system, within the constrains (volume and total energy) imposed by the system. A didactic explanation of enthalpy, entropy, and Gibbs free energy is given by Connors (2002) [6] and a quantitatively rigorous approach is described by Reif (1965) [1]. The above presented theory is also valid for the solubilization processes of hydrophobic solutes (and colloids) in aqueous solutions with the aid of micelles (surfactants) or microemulsions (stabilized submicron droplets of oil). The same can be told with the inverse, the solubilization of hydrophilic solutes in hydrophobic solvents.