The phenomenon of osmosis
occurs when pure water flows from a dilute saline solution through a membrane
into a higher concentrated saline solution.
<>The phenomenon of osmosis is
illustrated in Figure 1. A semi permeable membrane is placed between two
compartments. “Semi permeable” means that the membrane is permeable to some
species, and not permeable to others. Assume that this membrane is permeable to
water, but not to salt. Then, place a salt solution in one compartment and pure
water in the other compartment. The membrane will allow water to permeate
through it to either side. But salt cannot pass through the membrane.
As a fundamental rule of
nature, this system will try to reach equilibrium. That is, will try to reach
the same concentration on both sides of the membrane. The only possible way to
reach equillibrium is for water to pass from the pure water compartment to the
salt-containing compartment, to dilute the salt solution.
Figure 1 also shows that
osmosis can cause a rise in the height of the salt solution. This height will
increase until the pressure of the column of water (salt solution) is so high
that the force of this water column stops the water flow. The equilibrium point
of this water column height in terms of water pressure against the membrane is
called osmotic pressure.
If
a force is applied to this column of water, the direction of water flow through
the membrane can be reversed. This is the basis of the term reverse osmosis.
Note that this reversed flow produces pure water from the salt solution, since
the membrane is not permeable to salt. |
