Osmotic pressure

Osmotic pressure is the pressure required to stop solvent from crossing a semipermeable membrane into a solution. It depends on the concentration and nature of dissolved particles.

Osmosis: The net movement of solvent across a selectively permeable membrane toward the side with higher effective solute concentration. Osmotic pressure is the counterpressure that halts this net solvent movement.
Colligative property: A solution property that depends primarily on the number of dissolved particles rather than their chemical identity. Osmotic pressure is a colligative property in the ideal dilute limit.
Reverse osmosis: A separation process that uses applied pressure to drive solvent through a membrane against its natural osmotic direction. It requires applied pressure exceeding the feed solution’s osmotic pressure.
Tonicity: The tendency of a solution to cause a cell to gain or lose water, determined by nonpenetrating solutes. Tonicity describes cell-volume effects related to osmotic gradients across cell membranes.
Van ’t Hoff equation: A relation estimating dilute-solution osmotic pressure from absolute temperature and the concentration of dissolved particles. It quantifies how particle concentration and temperature set osmotic pressure in dilute solutions.
Activity (chemistry): An effective concentration that accounts for nonideal interactions in a chemical system. Solvent activity, rather than concentration alone, governs osmotic equilibrium in nonideal solutions.
Osmometry: The measurement of solution osmotic pressure or related particle-concentration properties. Osmometry determines osmotic pressure directly or uses it to infer molar mass.
Plasmolysis: The withdrawal of a plant cell’s protoplast from its wall after water loss in a hypertonic environment. It is a visible consequence of water leaving cells under an external osmotic gradient.
Semipermeable membrane: A barrier that permits some substances to pass while restricting others. Its selective permeability makes a pressure difference capable of opposing solvent flow.
Osmotic equilibrium: A state in which opposing solvent chemical potentials produce no net osmotic flow. The defining pressure is the pressure difference that establishes this state.

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