There are many salts like NaCI, KNO₃ and KI which are quite soluble in water. On the other hand, some salts like BaSO₄, PbSO₄, AND AgCI are very slightly soluble in water and are called sparingly soluble salts. Thus, if a sparingly soluble salt like AgCI is put in water, a saturated solution results and an equilibrium gets established between the ions of the dissolved salt and undissolved salt as :

                                                              AgCI(s)  <==>  Ag⁺ + CI ^-

Applying, the law of chemical equilibrium, we have

                                                           K  =  [ Ag⁺] [CI ]
                                                                    [ AgCI(s) ]
or                                   [ Ag⁺] [CI ^- ]  = K  X  [AgCI(s) ]

Since the concentration of a solid like AgCI(s) is always constant, no matter how much solid is present in contact with its solution, therefore, we have

                                        [ Ag⁺] [CI^- ]  =  K  X  constant  = K_sp

where K_sp is known as solubility product and the expression [ Ag⁺] [CI ^- ] is known is ionic product.

In general, for any sparingly soluble salt  A_xB_y which dissociates to set up the equilibrium

                                      A_xB_y   <==>    X Ay⁺ + yB^x-

the solubility product may be expressed as :

                                        Ksp  =  [ A ^y+ ]^x [ B^x- ]^y

where A^y+ and B^x- denote the positive and negative ions respectively while x and y represent the number of these ions in the formula of the electrolyte.

Hence, the solubility product a salt at a given temperature may be defined as the product of the concentration raised to a power equal to the number of times each ion occurs in the equation for the dissociation of the salt.

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