What is osmosis?
Osmosis is the way water moves through a semipermeable membrane from a weaker solution to a stronger one. The membrane has tiny gaps that let water molecules slip through but stop larger dissolved particles, so the two sides behave very differently.
On this diagram the left compartment holds a dilute solution — mostly water with a few orange particles — while the right compartment is packed with dissolved particles. A single membrane divides them.
Reading the diagram
The blue dashed arrows show the net direction of water movement, crossing the membrane from left to right. The red dashed lines mark the two liquid levels: the dilute side sits low, the concentrated side has climbed higher. The orange dots are the dissolved particles the membrane holds back.
Why it matters
Osmosis keeps water moving inside every living thing. It pulls water into plant roots, balances the fluid inside cells, and is the engine behind water filters and desalination. Whenever two solutions of different strength meet across a membrane, water starts to drift toward the stronger one.
1. Two solutions of different strength
The dilute side is mostly water with only a few dissolved particles; the concentrated side is crowded with them. This difference in particle count is the whole reason osmosis happens.
2. The membrane lets only water through
The semipermeable membrane has openings small enough for water molecules but too small for the dissolved particles. So water can cross in either direction, while the particles stay trapped on their own side.
3. Water crosses in both directions, but not equally
Water molecules constantly wander through the membrane both ways. On the concentrated side, the dissolved particles leave less room for water, so fewer water molecules find their way out. Traffic out of the dilute side is heavier than the return traffic.
4. The net flow points to the concentrated side
Because more water leaves the dilute side than comes back to it, water steadily moves toward the concentrated side, exactly as the blue arrows show. The particles cannot follow it, so the concentrated side simply gains water.
5. The level rises and pushes back
As water collects on the concentrated side, its level climbs. The taller column presses down harder and harder until it pushes water back through the membrane as fast as it arrives. At that balance the two-way flow is equal and the rise stops — even though the two concentrations are still different.
What is osmosis?
Osmosis is the movement of water through a semipermeable membrane toward the side with more dissolved particles. Only the water crosses; the dissolved particles stay put, so the more concentrated solution slowly gains water and rises.
Why does water move toward the concentrated side?
Both sides get the same push of water trying to cross, but the side crowded with dissolved particles has less room for water, so fewer water molecules leave it. The dilute side therefore loses more water than it gains, and water flows toward the concentrated side.
Can dissolved particles cross the membrane?
No. A semipermeable membrane lets small water molecules through but blocks larger dissolved particles. That is what makes the two sides behave differently and drives the flow in one direction.
When does osmosis stop?
It stops when the extra height on the concentrated side is just enough to push back as hard as the dissolved particles pull. At that balance point the two-way traffic of water becomes equal, even though the two concentrations are still different.
What is osmotic pressure?
Osmotic pressure is the extra push needed to stop water from crossing into the concentrated side. It grows with how many dissolved particles are present, which is why very salty or sugary water can pull water strongly through a membrane.