The Foundations of Chemistry

two solutions. Solvent molecules may pass through the membrane in either direction, but
the rate at which they pass into the more concentrated solution is found to be greater
than the rate in the opposite direction. The initial difference between the two rates is
directly proportional to the difference in concentration between the two solutions. Solvent
particles continue to pass through the membrane (Figure 14-16a). The column of liquid
continues to rise until the hydrostatic pressure due to the weight of the solution in the
column is sufficient to force solvent molecules back through the membrane at the same
rate at which they enter from the dilute side. The pressure exerted under this condition
is called the osmotic pressureof the solution.

Figure 14-16 (a) Laboratory

apparatus for demonstrating osmosis.

The picture at the right gives some

details of the process, which is

analogous to the transfer of solvent

into a solution through the space

above them (b). In (a) the solute

particles cannot pass through the

semipermeable membrane. In (b) the

solute particles cannot pass through

the vapor phase because they are

nonvolatile.

14-15 Osmotic Pressure 571

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 14.9, Colligative Properties (3):

Osmosis.

Thistle tube

Level at start

Sugar solution

Rubber band

Membrane Water

Solution level

after osmosis

has progressed

for a time

Sugar

molecules

cannot pass

through

membrane

Some H 2 O

molecules

pass out of

the solution

Water molecules pass

in through membrane

(a)

(b)

Start Later

Start Later

Solution Solvent

Solution