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Osmosis
Article by: Francis J. Johnston, Department of Chemistry, University of Georgia, Athens, Georgia
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The selective passage of solvent molecules
through a semipermeable membrane from a
dilute solution to a more concentrated one. A
semipermeable membrane allows the passage of
solvent molecules but blocks the passage of sol-
ute molecules. The movement of water by osmosis
through cell membranes is essential for all physiologi-
cal processes. Osmosis also has diverse experimental
and industrial applications.
Mechanism of Osmosis
The phenomenon of osmosis may be observed by
dividing a container into two equal volumes with a
semipermeable membrane. As shown in Fig. 1, the
left side contains pure water, the right side contains
a dilute glucose solution, and the water levels are
equal. Over time, the level on the right rises as water
flows from the left to the right side. As the solution
level on the right increases, pressure forces water
back through the membrane. Eventually, the system
equilibrates, such that the rates of the forward and
reverse water flow through the membrane are equal
and the level on the right stops rising. The osmotic
pressure is the pressure required to reach equilibrium
(that is, to stop osmosis) and is a characteristic of the
solution (Fig. 2).
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Key Concepts
• Osmosis is the transport of a solvent through a semipermeable membrane that separates two solutions of differing
solute concentration.
• During osmosis, the solvent moves from the solution that is lower in solute concentration to the solution that is higher
in solute concentration.
• Osmotic pressure describes the minimum pressure that, when applied to the solution phase, prevents the solvent from
passing through a semipermeable membrane into the solution.
• The decrease in Gibbs free energy accompanying the dilution of the solution is important in driving osmosis.
• Reverse osmosis involves movement of solvent from a higher solute concentration to a lower solute concentration.
This happens when hydrostatic pressure is greater than osmotic pressure.
Fig. 1: Osmosis process.
(Credit: McGraw Hill)
Osmotic Pressure
Diffusion is the process by which particles or molecules spread out and
move from an area of high concentration to an area of low concentra-
tion. Because the solute cannot cross the semipermeable membrane,
but the solvent can, the solvent will tend to cross the membrane to
equalize the concentration of the solute on the two sides of the mem-
brane in the process of osmosis. If the final result of a reaction is a more
stable solution, then the molar free energy is positive, and the reaction
is likely to happen. The molar free energy of a solvent in a solution is
