Osmotic pressure is the minimum pressure that must be applied to a solution to stop the flow of solvent molecules into it through a semipermeable membrane.
Breaking it down:
- Osmosis is the natural process where solvent molecules (like water) move from a region of low solute concentration (dilute solution or pure solvent) to a region of high solute concentration (concentrated solution) through a semipermeable membrane.
- This flow continues until equilibrium is reached.
- To prevent this movement, an external pressure is applied on the solution side. That pressure is called osmotic pressure.
Key Points:
- It depends on the concentration of solute particles in the solution (not their type).
- It is a colligative property – meaning it depends only on the number of solute particles, not their identity.
- Higher solute concentration → higher osmotic pressure.
Examples in real life:
- Plant roots absorb water from soil by osmosis.
- Red blood cells swell or shrink depending on the osmotic pressure of the surrounding fluid.
- Reverse osmosis (RO water purifiers) apply external pressure greater than osmotic pressure to force water through a membrane, leaving impurities behind.
In short: Osmotic pressure is the pressure required to stop osmosis, and it plays a vital role in biology, medicine, and water purification.