Step-by-step explanation:
A solution in which more of a solute is not able to dissolve and retains undissolved particles at the bottom of the vessel is known as an unsaturated solution. The particles of solute stop to dissolve further because an equilibrium is maintained between solute and solvent.
Whereas a solution in which solute is able to dissolve completely without any traces of undissolved particles is known as a saturated solution. The solute particles keep on dissolving until an equilibrium has reached.
Therefore, we can conclude that when an equilibrium exists between the dissolved and the undissolved solute in a solution the solution must be unsaturated.