Reverse osmosis is a semipermeable membrane that will pass some atoms or
molecules but not others. The best common example of a semipermeable
membrane would be the lining of your intestines. The membrane allows
passage of water molecules but not salt molecules. One way to understand
osmotic pressure would be to think of the water molecules on both sides of
the membrane. They are in constant Brownian motion. On the salty side, some
of the pores get plugged with salt atoms, but on the pure-water side that
does not happen. Therefore, more water passes from the pure-water side to
the salty side, as there are more pores on the pure-water side for the water
molecules to pass through. The water on the salty side rises until one of
two things occurs. The salt concentration becomes the same on both sides of
the membrane (which isn't going to happen in this case since there is pure
water on one side and salty water on the other). The water pressure rises as
the height of the column of salty water rises, until it is equal to the
osmotic pressure. At that point, osmosis will stop.

Osmosis, by the way, is why drinking salty water (like ocean water) will
kill you. When you put salty water in your stomach, osmotic pressure begins
drawing water out of your body to try to dilute the salt in your stomach.
Eventually, you dehydrate and die.

In reverse osmosis, the idea is to use the membrane to act like an extremely
fine filter to create drinkable water from salty (or otherwise contaminated)
water. The salty water is put on one side of the membrane and pressure is
applied to stop, and then reverse, the osmotic process. It generally takes a
lot of pressure and is fairly slow, but it works.

Here is one more for you to debate. Take distilled, tap water, and RO
water. Put all 3 glasses in a microwave and see if they boil at the same
time. Pure distilled water will not boil a 212 it will become super heated.
The reason distilled doe not boil is there are no impurities in the water.
Tap and RO will boil sat the same time they both contain impurities. To
make the distilled water boil just add a little salt, sugar, or any other
impurity. Be careful it may explode back on you leaving a nasty burn.

Jack

Olly wrote in message ...
Watermakers remove almost all the TDS - supposedly 99+% - so the water
is like distilled. Is it unhealthy to drink nothing but distilled
water - no mineral content - for years while cruising.