NaCl(aq), NaCl(aq), C C C C C C C was the equation that the chemist was thinking of when he knew he needed a vacation. What did it mean?

Saline, saline, over the seven Cs

How does reverse osmosis purify water?

If you go to an appliances shop, you'll notice many brands of water purifiers that work by reverse osmosis. You may have one at home too. But how exactly does reverse osmosis work?

What is osmosis?

Osmosis is a special form of diffusion. So what's diffusion, then? Well, to know it, try out an experiment. Take glass of water, add a few drops of fruit juice concentrate and let them settle at the bottom. Don't stir or swirl. Leave the glass on the table, and watch it over time. You will see that the drops slowly mix with the water, and the whole glass becomes coloured. That's diffusion.

So what's osmosis? Try another experiment. Place a few dried peas in a cup and fill it with purified water. In another cup, pour in some brine (a thick solution of salt) and put a few fresh peas in it. Leave for a few hours. What happens?

Peas are made of cells like all living organisms. And each cell is covered with a semi-permeable membrane. It allows water to pass through, but not dissolved substances. If the concentration of sugar or salt differs on either side of the membrane, water molecules will travel from side with less sugar to the side with more. Here's a video* explanation:

Osmosis explains why the fresh peas shrivel in the salt solution: water diffuses from the peas to the brine. It's also why the dried peas swell up, water diffuses into the peas. Now that we know what osmosis is, let's understand reverse osmosis.

Reverse osmosis

As the name says, it's the reverse. Water diffuses from where there are more dissolved substances to where there are fewer. This does not happen naturally, but can be made possible by applying pressure. So you get pure water from a source of water that has many contaminants in it.

reverse osmosis drawing

That's why reverse osmosis is a popular method for purifying drinking water. The ones we get commercially use a strong semi-permeable membrane that can withstand pressure. Usually no more pressure is necessary than the pressure already in the water pipe (when water comes to your kitchen from your building's overhead water tank, it is already under a lot of pressure). Reverse osmosis is also used to make drinking water from sea water in places where a source of fresh water is scarce. And it is an important process for purifying many chemicals on an industrial scale.

*Video titled "khanacademy - February 08, 2010 - Diffusion and Osmosis" sourced from

Tags :     Everyday Chemistry     Water     reverse osmosis     semipermeable membrane    

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