The solubility of ionic solids in water differs to a great extent. Some of the ionic solids are so soluble that they are hygroscopoic in nature and absorb even water from the atmosphere. On the other hand there are substances which have o less soluble that these are regarded as insoluble. There are some ionic solids which have solubility in between these cases. On the basis of solubility, we can classify the salts into three main categories:
Category I: The salts which are soluble and there solubility > 0.1 M
Category II: The salts which are slightly soluble and there solubility is between 0.01 M and 0.1 M
Category III: The salts which are sparingly soluble and there solubility < 0.01 M
Let us consider the equilibrium between the sparingly soluble salts and its saturated solutions.
There are many compounds such as lead chloride (PbCl2), silver chloride (AgCl) etc. which are very slightly soluble in water. These substances are called sparingly soluble salts. When these sparingly soluble salts are dissolved in water, equilibrium is established between the undissolved solid salt and ions of the dissolved salt. For example:
For a sparingly soluble compound like AgCl, the following equilibrium occurs between the undissolved solid salts and the silver and chloride ions in the saturated solution:
AgCl (s) ⇌ Ag+ (aq) + Cl- (aq)
Applying the law of chemical equilibrium, we will get:
K = [Ag+] [Cl-] / [AgCl]
K x [AgCl] = [Ag+] [Cl-]
The concentration of the pure solid substance remains constant. Therefore, the concentration of solid AgCl in the solid state i.e. [AgCl] is constant at a particular temperature, no matter how much solid is present in contact with the solution. It follows:
K x constant = Ksp = [Ag+] [Cl-]
Where Ksp is known as solubility product constant or solubility product. Since the solubility of a salt usually varies widely with temperature, the numerical value of Ksp for a salt changes with temperature.
Generally, if the sparingly soluble salt, Ax By is in equilibrium with saturated solution of its ions, then
Ax By = x Ay+ + y Bx-
Where Ay+ and Bx- denote the positive and negative ions respectively and x and y represent the number of these ions in the formula of the electrolyte.
The solubility product expression is:
Ksp = [Ay+] x [Bx-] y
Hence, the solubility product of a salt at a given temperature is equal to the product of the concentrations of its ions in the saturated solution, with each concentration term raised to the power to the number of ions produced on dissociation of one mole of the substance.