Step 1: Start from the pressure balance. Every solution has a characteristic osmotic pressure \( \pi \). Applying exactly \( \pi \) on the solution halts osmosis; this is the turning point.
Step 2: Push the pressure above \( \pi \). Now the applied pressure overpowers the osmotic driving force and squeezes solvent out of the solution and across the membrane into the pure-solvent compartment. Running osmosis backwards like this is reverse osmosis.
Step 3: Because only solvent (water) crosses while ions are held back, reverse osmosis is the standard industrial method for purifying and desalinating water on a large scale.
Step 4: The membrane must be man-made and robust enough to bear the high pressure. Cellulose acetate film serves this purpose as an artificial semipermeable membrane; copper ferrocyanide deposited in a porous pot is another classic laboratory example.