Solubility

The maximum amount of a solute that can be dis­solved in 100 g of solvent at a given temperature (usually 25°C) is known its solubility at that temperature.

         

          The solubility of a solute in a liquid depends upon the following factors ­

          (a) Nature of the solute i.e. ionic or covalent (polar or non-polar).

          (b) Nature of the solvent i.e. polar with high dielectric constant or non-polar.

           (c) Temperature - Usually the solubility of the solute increases with increase of temperature (e.g. KNO3, NH4Br) but in some cases increase in solubility is negligible (e.g. NaCl) and in cases of some salts (e.g. Na2SO4 and CeSO4 solubility decreases with increase in temperature).

SOLUBILIY OF SOLIDS IN LIQUIDS

Some solids like sodium chloride and sugar dissolve readily in water but do not dissolve in liquids like benzene. While some other solids like naphthalene and anthracene dissolve  readily in benzene but they do not dissolve readily in water benzene but sodium chloride and sugar do not. The general rule for the solubility of solutions is that like dissolves like. For example, Polar, ionic substances are soluble in polar solvents, while non polar solutes are soluble in non polar solvents. For example, alcohol and water, which are both polar, can form a solution and iodine and carbon tetrachloride, which are both non polar, make a solution. However, iodine will not readily dissolve in polar water.

When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is called as dissolution. Some solute particles in solution collide with the solid solute particles and get separated out of solution. This is known as crystallisation. When these two processes start occuring at the same rate, the number of solute particles going into solution will become equal to the solute particles separating out. This is a state of dynamic equilibrium.

At this stage, the concentration of solute in solution will remain constant under the given temperature and pressure. The 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.

Effect of temperature

The solubility of a solid in a liquid depends on temperature. Consider the equilibrium between solution and solvent. This 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 rise 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 solubility of solids in liquids. It is so because solids and liquids are highly incompressible and practically remain unaffected by changes in pressure.

SOLUBILIY OF GASES IN LIQUIDS

Many gases dissolve in water. Oxygen dissolves only to a small extent in water. It is this dissolved oxygen which sustains all aquatic life. On the other hand, hydrogen chloride gas (HCl) is highly soluble in water.

Solubility of gases in liquids is greatly affected by pressure and temperature. The solubility of gases increase with increase of pressure. Henry was the first to give a quantitative relation between pressure and solubility of a gas in a solvent which is known as Henry’s law.