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To understand "reverse osmosis", it is probably best to start
with normal "osmosis" first. According to Merriam-Webster's
Collegiate Dictionary, Osmosis is the "movement of a solvent through
a semipermeable membrane (as of a living cell) into a solution of higher
solute concentration that tends to equalize the concentrations of solute
on the two sides of the membrane". That's a mouthful. To understand
what it means, this picture is helpful:
On the left is a beaker filled with water, and a tube has been half-submerged
in the water. As you would expect, the water level in the tube is the same
as the water level in the beaker. in the middle figure, the end of the tube
has been sealed with a "semi-permeable membrane" and the tube
has been half-filled with a salty solution and submerged. Intially the level
of the salt solution and the water are equal, but over time something unexpected
happens - the water in the tube actually rises. The rise is attributed to
"osmotic pressure".