Solubility of a Solid in a Liquid
Every solid does not dissolve in a given liquid. While sodium chloride and sugar dissolve readily in water, naphthalene and anthracene do not. On the other hand, naphthalene and anthracene dissolve readily in benzene but sodium chloride and sugar do not. It is observed that polar solutes dissolve in polar solvents and nonpolar solutes in nonpolar solvents. In general, a solute dissolves in a solvent if the intermolecular interactions are similar in the two or we may say like dissolves like. When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is known as dissolution. Some solute particles in solution collide with the solid solute particles and get separated out of solution. This process is known as crystallization. A stage is reached when the two processes occur at the same rate. Under such conditions, a number of solute particles going into the solution will be equal to the solute particles separating out and a state of dynamic equilibrium is reached.
Solute + Solvent --> Solution
At this stage, the concentration of solute in solution will remain constant under the given conditions, i.e., temperature and pressure. A similar process is followed when gases are dissolved in liquid solvents. Such a solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. An unsaturated solution is one in which more solute can be dissolved at the same temperature. The solution which is in dynamic equilibrium with undissolved solute is the saturated solution and contains the maximum amount of solute dissolved in a given amount of solvent. Thus, the concentration of solute in such a solution is its solubility. Earlier we have observed that solubility of one substance into another depends on the nature of the substances. In addition to these
variables, two other parameters, i.e., temperature and pressure also control this phenomenon.
Effect of temperature The solubility of a solid in a liquid is significantly affected by temperature changes. Consider the equilibrium represented by the equation . This, being dynamic equilibrium, must follow Le Chateliers Principle. In general, if in a nearly saturated solution, the dissolution process is endothermic (Δsol H > 0), the solubility should increase with rising in temperature and if it is exothermic (Δsol H > 0) the solubility should decrease. These trends are also observed experimentally.
Effect of pressure Pressure does not have any significant effect on the solubility of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure