Question

Why must some solid remain in contact with a solution of a sparingly soluble salt in order to ensure equilibrium? 8.

Answer 1

In simple words we can say that,

Equilibrium is a dynamic process. Removing some solid would cause the equilibrium of the solution to shift to the left and produce more solid from the dissolved solution to reestablish equilibrium.

Lets look at a simple example with say, silver chloride

The equilibrium we are talking about is AgCl(s) <---> Ag+(aq) + Cl-(aq)

At equilibria these will be at a certain balance, controlled by the equilibrium constant Kc

But what would happen if you removed the solid AgCl? The condition for Kc would not longer be true and silver ions and chloride ions would have to react to form AgCl solid. This is why solid must be present.

The other reason is that temperatures rarely remain constant, and the presence of solid also allows the equilibria to adjust to the small temperatures changes. {Kc varies with temperature]