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Could a filter be used to isolate salt from sea water? Could we say that the water molecules are smaller than the molecules of salt dissolved in the water. I have no relation to chemistry or physics, so pardon the question if it's too naive.

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Yes, this is possible and is a process called reverse osmosis. It is based on a membrane which lets water pass but not the salt ions (and other water contaminants). From Wikipedia:

The membranes used for reverse osmosis have a dense layer in the polymer matrix -- either the skin of an asymmetric membrane or an interfacially polymerized layer within a thin-film-composite membrane -- where the separation occurs. In most cases, the membrane is designed to allow only water to pass through this dense layer, while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–82 bar (600–1200 psi) for seawater

Water desalination features prominently among applications of reverse osmosis:

The largest and most important application of reverse osmosis is to the separation of pure water from seawater and brackish waters; seawater or brackish water is pressurized against one surface of the membrane, causing transport of salt-depleted water across the membrane and emergence of potable drinking water from the low-pressure side.

Reverse osmosis

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  • $\begingroup$ Thank you for your detailed feedback. Just wondering if this technique could be used on a large scale and if it requires any specially costly components or materials? thanks again. $\endgroup$ – NoChance Aug 29 '12 at 11:14
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    $\begingroup$ @EmmadKareem, reserve osmosis is essentially how all of the oil-producing countries in desert lands produce a significant portion of their water. Yes, it is done at large scales. $\endgroup$ – bobthejoe Aug 29 '12 at 11:35
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    $\begingroup$ @EmmadKareem Reverse osmosis is rarely used outside of desert countries because the large amount of energy needed makes it significantly more expensive than other sources of fresh water (when available). Efficiency of the process is increasing; but about a year ago I read an article stating that current designs were approaching engineering/thermodynamic limits and additional large gains were unlikely. arstechnica.com/science/2011/08/… $\endgroup$ – Dan Neely Aug 29 '12 at 14:46
  • $\begingroup$ @DanNeely It may be currently uncommon to use desalination outside of desert countries, but it isn't unknown: London has a desalination plant. And this article from the register argues the economics are favourable even there compared to the cost of reducing leakage. $\endgroup$ – matt_black Oct 28 '12 at 14:11
  • $\begingroup$ @matt_black It's not an either or decision. Leaking pipes wash holes into the underground of your city. That will be very expensive on the long run. $\endgroup$ – Karl Dec 5 '16 at 14:30

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