(eli5) How is water treated and what does it mean when a water bottle says “purified by reverse osmosis” ?

Question

(eli5) How is water treated and what does it mean when a water bottle says “purified by reverse osmosis” ?

In: Chemistry

Answer 1

There are certain thin films (membranes) that have holes (pores) small enough to let water molecules through but not let larger ones through. If the membrane is between pure water and water with solutes (dissolved molecules in the water), pure water will naturally move through the pores into the impure water. Some water molecules will move both ways, but a much greater number of them will move from the pure side to the other side. This is called osmosis. If the impure water side is pressurized, however, water will move from the impure side through the pores in the membrane to the side with the pure water. This is called reverse osmosis.