When two solutions of different concentrations are separated by semi permeable membrane (mostly made of cellulose acetate,poly methycrylate and poly amide polymers). The solvent particles move from lower concentration to higher concentration through the membrane. This process is called Osmosis and the pressure difference develop between thelower concentration side and higher concentration side is called osmotic. osmotic pressure Po=P1-P2. The particle moves through the membrane because of this osmotic pressure.
When an external hydrostatic pressure (15 to 40 Kg) which is excess than the osmotic pressure which is applied on the higher concentration side, the process of osmosis get reverse and the molecules forced to move from the higher concentrated side to lower concentration side through the membrane. This process is called reverse Osmosis.
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