When does water stop moving across a membrane?

Water stops moving across a membrane when the concentrations of the solutions on both sides of the membrane reach equilibrium. Once they reach equilibrium, the solutions are called isotonic solutions.

If a cell is placed in a solution that has a higher concentration of a dissolved substance than the solution inside the cell, water moves across the membrane to the outside of the cell and causes the cell to shrink. If a cell is placed in a solution that has a lower concentration of a dissolved substance than the solution inside the cell, water flows into the cell and causes the cell to swell.