When two solutions separated by a selectively permeable membrane reach osmotic equilibrium, water molecules still move across the membrane, but there is no net movement of water in either direction. The overall concentrations of water and non‑permeating solutes on both sides become effectively equal in terms of osmotic effect, so the volumes and pressures on each side stay stable.

Quick Scoop

  • At osmotic equilibrium:
    • Water crosses the membrane in both directions at equal rates.
* There is no _net_ gain or loss of water on either side.

* Osmotic pressure difference across the membrane is zero (or exactly balanced by hydrostatic pressure, if present).

* The solutions are isotonic to each other in terms of osmotic effect, even if their exact solute identities differ.

What “osmotic equilibrium” really means

  • A selectively permeable membrane lets water pass but not certain solutes, so water moves from the side with higher water concentration (lower solute) to the side with lower water concentration (higher solute).
  • Osmotic equilibrium is reached when:
    • The water concentration difference is gone or
    • Any remaining tendency for water to move is exactly counterbalanced by hydrostatic pressure (like a water column pushing back).

At this point, individual water molecules keep diffusing randomly, but for every molecule going left, another goes right, so the net flux is zero.

What happens on each side of the membrane?

  • Water levels / volumes
    • If one side initially drew in water (hypertonic side), its volume increases until the pressure or dilution stops further net inflow.
* Once equilibrium is reached, volumes remain constant unless conditions change (e.g., solute is added or removed).
  • Solute concentrations
    • Non‑permeating solutes stay on their original sides; they do not cross the membrane.
* Water movement changes their concentrations until the osmotic effect on both sides balances out (isotonic situation).

Common classroom picture

Many textbooks illustrate this with a U‑tube:

  1. One arm has a higher solute concentration (hypertonic), the other a lower solute concentration (hypotonic).
  1. Water moves into the hypertonic side, raising the liquid level there.
  1. Eventually, the raised column of water exerts enough hydrostatic pressure to oppose further net water entry, and osmotic equilibrium is reached.

At that moment, the heights of the two columns stop changing, but water molecules continue to cross the membrane both ways at equal rates.

In cells and real biology

  • Animal cells in an isotonic environment (same effective solute concentration inside and out) are at osmotic equilibrium with their surroundings, so they neither swell nor shrink.
  • Cells actively regulate solute content (osmoregulation) to keep near osmotic equilibrium with their environment and avoid bursting or shriveling.

TL;DR: When two solutions separated by a selectively permeable membrane reach osmotic equilibrium, water still moves both ways, but there is no net movement, solute concentrations are balanced in osmotic effect, and volumes and pressures stay stable.

Information gathered from public forums or data available on the internet and portrayed here.