In normal osmosis, solvent flows spontaneously from a region of lower solute concentration to higher solute concentration through a semipermeable membrane. In reverse osmosis, an external pressure exceeding the osmotic pressure is applied on the solution side. This reverses the natural direction of solvent flow, pushing pure solvent (water) from the concentrated solution side through the membrane to the other side, leaving dissolved salts behind. The membrane is selectively permeable—it allows water molecules to pass but blocks dissolved ions and larger molecules. This is the basis of desalination plants. Applying pressure exactly equal to osmotic pressure would only stop osmosis (equilibrium), not reverse it; the applied pressure must be strictly greater.