Page 14 - Curriculum Visions Dynamic Book

Salt and osmosis
Osmosis is one of the most important processes in the natural world. It describes the way that
a liquid can pass through some materials but substances dissolved in it may be held back. Osmosis plays a vital part in industrial chemistry and is used to act as a natural “sieve”. This
kind of sieve is known by scientists as a “semipermeable membrane”. A common example of a semipermeable membrane is human
skin. The tissue walls of many of
our internal organs are also semipermeable.
If a concentrated solution and a dilute solution are put together, they will mix until the whole solution is the same concentration. (For example, a dye will spread through a liquid). The semipermeable membrane works because the molecules of the liquid part of a solution are small, but the molecules of the substances dissolved in the liquid are large. The membrane has holes in it just
large enough to allow
the liquid molecules
to pass through easily,
but just too small to
allow the molecules
of the dissolved
substances.
Desalination
A process called reverse osmosis is used to remove salts from impure natural water. This is how desalination plants work.
Salty water is pumped through tubes made
of a semipermeable membrane (often a kind of synthetic rubber). Pure water is collected from outside the membrane.
 This is a desalination plant in the Middle East, a region naturally very short of water but with enough oil reserves to use an energy-intensive method of collecting fresh water.
The principles involved in osmosis are demonstrated here using salt.
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