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 crystallisation . A stage is reached when the two processes occur at the same rate.
Under such conditions, number of solute particles going into 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. 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 equation .
. This, being dynamic equilibrium, must follow Le Chateliers Principle. In general, if in a nearly saturated solution, the dissolution process is endothermic ( D sol H > ), the solubility should increase with rise in temperature and if it is exothermic ( D sol H < ) the solubility should decrease. These trends are also observed experimentally.
Effect of pressure Pressure does not have any significant effect on solubility of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure. Many gases dissolve in water. Oxygen dissolves only to a small extent in water.