Solubility
Text 30 A Solubility - раздел Образование, Part One THE PREDICATE While There Arc Many Pairs Of Substanccs Which, Like Water And Tfthyl Alcohol...
While there arc many pairs of substanccs which, like water and tfthyl alcohol, can be mixed in any proportions to form homogeneous solutions, it is a matter of common experience that the capacity of a solvent to dissolve a given solute is often limited. When a solvent placcd in contact with an cxccss of solute attains and maintains a constant concentration of solute, the solute and solution are at equilibrium, and the solution is said to be saturated. The solubility of a substance in a particular solvent at a given temperature is the concentration of the solute in the saturated solution. In other words, the solubility of a solute is the dissolved concentration characteristic of the state of equilibrium between die solute and the solution. It is difficult to overemphasize the importance of the conccpt of solubility to chcmistry; it is the basis of innumerable laboratory and industrial processes that prepare, separate, and purify chcmicals, and is the controlling factor in a variety of geological and other natural phenomena. The solubility of a substance in a particular solvent is controlled principally by the nature of the solvent and solute themselves, but also by the conditions of temperature and pressure. To analyse these factors, wc shall first limit ourselves to the case of ideal solutions.
The liquids thai form an ideal solution arc always miscible in any proportions and, thus, have infinite solubility in cach other. The reason for this is easy to sec if wc recall two facts. First, limited solubility and a saturated solution result only when a solute and its solution reach equilibrium. Sccond, the equilibrium state is a compromise between a natural tendency toward minimum energy and maximum molccular chaos. Now, the mixing of two ideal liquids is always accompanicd by an increase in entropy or molccular chaos, bccausc in the solution, the solute molcculcs arc spread randomly throughout the solvent, rather than being nearly closest packed as they arc in the pure solute. That is, even if wc could loeate one solute molcculc in solution, wc could not prcdict what the identity of its nearest neighbours was, as wc could, if the molecule were in the pure solute phase. Consequently, the solution has a higher entropy than the pure solvent and solute, and the tendency toward maximum molccular chaos favours the mixing of the two liquids. Moreover, the fact that there is no energy change in the mixing proccss means that the tendency toward minimum energy docs not restrict the solution proccss. Consequently, the two liquid components of an ideal solution can mix in any proportion.
