Final answer:
An isotonic solution remains unchanged when a cell with the same osmolarity is placed in it, even if a permeable solute can move across the membrane since there is no net movement of water due to the initial equilibrium.
Step-by-step explanation:
When a cell is placed into a solution that has the same osmolarity or is isotonic, and one of the solutes in this solution, which is not present in the cell, can move across the membrane, the behavior of the solution will depend on the solute movement. Since the extracellular fluid and the cell's interior are isotonic, there is no net movement of water. However, if the solute can move across the membrane, then it will do so until it reaches equilibrium on both sides. As this solute moves, the osmolarity of the inside and outside of the cell may change, potentially altering the initial isotonic condition. Despite this, initially, the solution becomes unchanged because it was already at osmotic equilibrium with the cell.