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Osmosis (/ɒzˈmoʊ.sɪs/) is the movement of solvent (liquid) molecules through a membrane, from one solution to another, which happens without an outside force.

Solvent will move to the side that has a higher concentration of solutes, or dissolved particles — and thus a lower concentration of solvent. This happens because the membrane is selectively permeable: the solvent can pass through, but the solute cannot. Solvent molecules move randomly, and so the concentrations on both sides become more equal.