Question

What is a solubility test?

Answer 1

Solubility product (${{K}_{sp}}$) of a salt may be defined as "the product of the concentrations of the ions of the salts in its saturated solution at a given temperature raised to the power the number of ions produced by the dissociation of one mole of the salt".

Complete step-by-step answer: Certain salts like $NaCl,CuS{{O}_{4}},AgN{{O}_{3}}$, etc are highly soluble in water but at the same time the salts such as $AgCl,PbS{{O}_{4}},BaS{{O}_{4}}$etc are only sparingly soluble. The latter readily form a saturated solution while the former do so only at a high concentration which means that a large amount of the salt dissolves in water before it settles to the bottom of the container indicating a saturated solution. Since the salts are strong electrolytes, whatever amount goes into the solution exists as ions. Therefore there is a state of equilibrium between the salt present at the bottom of the container and the ions formed in solution. This is called solubility equilibria.
Constant of solubility product (${{K}_{sp}}$)
Different salts dissolve to different extent in a solvent, generally water. Some salts are highly soluble while the others dissolve to a smaller extent. At the same time salts like $PbS{{Os}_{4}},BaS{{O}_{4}}$etc. are only slightly soluble in water. Now a problem arises as to how to compare the relative solubilities of the salts. This is done with the help of a constant known as solubility product (${{K}_{sp}}$). Let us find the K value for silver chloride which is a sparingly soluble salt.
In general, for the saturated solution of a sparingly soluble salt like A, B, in water, the solubility:
${{A}_{x}}{{B}_{y}}(s)\rightleftharpoons x{{A}^{y+}}(aq)+y{{B}^{x-}}(aq)$
Here, A and B represent the positive and negative ions while x and y indicate the number of such ions.

Note: It is to be noted that solubility product is only applicable to sparingly soluble salts, as other salts dissociate and ionize completely.