(eli5) How is water treated and what does it mean when a water bottle says “purified by reverse osmosis” ?
In: Chemistry
Reverse osmosis is where water is cleaned by either pushing it or pulling it through a membrane. The membrane has tiny holes that are large enough for water to pass through but are too small for dirt and other particles.
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.
Soluble(mixable) solutions try to even out their distribution if allowed to mix. So if you poured pure water into a cup of equal amount of salt, it won’t stay pure water on top and salt on bottom, it’ll mix until it’s evenly 50/50 water/salt (assuming it stays heated enough to keep crystals from forming, but we’ll not get into that.)
The same remains true if they are mixing through a semipermeable (lets some things through but blocks others) membrane. The water will try to move into the side of the membrane with less salt unless blocked by the membrane, *even if* it results in increased pressure on one side of the membrane. That’s osmosis.
The analogy I use is a tank with a glass wall. On one side of the wall are big fish, on the other side are small fish. The big fish don’t care about the small fish, they only want to spread out from each other. The same is true about the small fish, they only want to spread out until each is more or less equal distant from each other. The glass wall is replaced with a net that allows small fish through but blocks the big fish (semipermeable). A portion of the small fish move into the side with the big fish until they are equal distant from each other again, *even if* this means overcrowding that side of the tank a bit (increases pressure). They just want to spread out.
Reverse osmosis is kinda the opposite. So much pressure is put on a semipermeable membrane that it forces that which can pass through it to the other side while blocking that which can’t pass through it. Honestly I think a filter is a more accurate way to say it. To go back to my earlier analogy, A mix of big fish and small fish is squeezed against the net. The only thing that pops out the other side of the net are small fish (purified water).
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