When two solutions separated by a selectively permeable membrane reach osmotic equilibrium, the net movement of water molecules between the two solutions stops. This occurs because water moves from the side with higher water concentration (lower solute concentration) to the side with lower water concentration (higher solute concentration) until the concentrations of solute on both sides become equal or the osmotic pressure is balanced by other forces such as hydrostatic pressure. At osmotic equilibrium, water molecules continue to move in both directions across the membrane, but at equal rates, stabilizing the solutions' concentrations on either side.
In detail:
- Osmosis is the net movement of solvent (usually water) through a selectively permeable membrane from a region of higher solvent concentration to a region of lower solvent concentration, aiming to equalize solute concentrations.
- The selectively permeable membrane allows water to pass but blocks solutes.
- Osmotic pressure is the pressure needed to prevent water from moving across the membrane due to osmosis, and equilibrium is reached when this pressure balances the concentration gradients.
- At equilibrium, there is no net flow of water, although water molecules still move back and forth equally.
Thus, reaching osmotic equilibrium means the solutions on both sides have equalized their solute concentrations to the extent allowed by the membrane's permeability, stopping net water movement while maintaining dynamic molecular exchange.
