Question

Briefly explain how aqueous base can cause some water-insoluble compounds to become water-soluble. (b) Answer the same question for aqueous acid.

Answer 1

Solution:

1) Addition of aqueous base increases the OH- concentration and therefore pH of the solution will increased. Thus, a water-insoluble compounds becomes soluble on addition of aqueous base.

For example: Solubility of sulfides (ZnS)

ZnS + 2OH-= Zn(OH)2 + S--

Solubility of sulfides increases with addition of aqueous base.

According to Le-Chatelier's principle, addition of OH- decreases the concentration of Zn2+ ion, hence equilibrium is shifted to forward direction and solubility increases.

2) Addition of aqueous acid (H3O+) causes an increase of H+ concentration and hence decreases the pH of the solution and solubility increased.

For example: Solubility of CaCO3

If you decrease the pH by adding H3O+ ion in CaCO3, then the added H3O+ ion will react with the OH- ions and form water. Thus, OH- ions are removed from solution. According to Le Châtelier's Principle, the position of equilibrium moves to the right to replace the  OH- ions. The solubility of the CaCO3 increases.