Which water-purification process uses pressurized water applied to a semipermeable membrane?

• A. Reverse osmosis
• B. Distillation
• C. Filtration
• D. Ion exchange

Answer: (A)

This question hinges on how a semipermeable membrane can separate water from dissolved substances when pressure is applied. In reverse osmosis, external pressure is used to push water through a semipermeable membrane that blocks salts and other contaminants. The pressure drives the flow opposite to natural osmosis, so clean water passes through while most dissolved ions stay on the other side. That combination of a semipermeable membrane and applied pressure is what makes reverse osmosis distinct.

Distillation relies on boiling and condensation to separate water from impurities, not a membrane and pressure-driven flow. Filtration uses a barrier to remove particles, but not through a semipermeable membrane under pressure in the osmotic sense. Ion exchange uses resin beads to swap undesirable ions for others, again not membrane-based.