In the last InfoChem we discovered that The Mariner in the film Waterworld could not make his urine drinkable simply by filtering it because this wouldnt remove all the salts. So how could he do this?
On boats and on small islands where fresh water is very precious, a technique called reverse osmosis is used to purify water. Osmosis is the movement of atoms and ions between solutions of different concentrations, seperated by a thin porous membrane. One classic experiment is a glass U-tube separated bottom-centre by a thin membrane. One side has a salty solution, the other a similar volume of pure water. Osmosis takes place and makes the pure water appear to 'dilute' the salty one. Water flows from one side to the other and the height of the column of the salty water rises a little owing to this extra 'ostomtic' pressure.
It's possible to reverse this process by using a pump to force water back against the natural osmotic pressure. The impure water is forced back through the membrane and collected from the pure water section - this is reverse osmosis.
The photo shows a home-made reverse osmosis unit (in white), hand pump (green) in the background and collecting cup. This really is like the one shown at the start of Waterworld. So Hollywood did think about and wanted the right prop for the film. Impure water is fed into the top of the apparatus and then sealed shut. Next a hand pump is used to drive the water through the membrane (a Gore-Tex like material). The membrane will only pass very small molecules such as H2O and (because of their charges) ions are blocked very efficiently. The more impurities in the water, the more pressure you need to purify it using this process.
So this would explain why The Mariner did not use sea water - his hand pump would probably not have supplied enough pressure. Since these units are often used in caravans and boats it would be just the sort of thing that The Mariner would have been able to find, clean-up and get working.
How teachers can use these articles in a lesson
Why Hollywood Science
Open University Hollywood Science web site
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