pressure is particularly easy to understand on the
basis of escaping tendencies.
It is illustrated by the diagram opposite, which shows
a single glass tube with one end covered by a semipermeable
membrane, and immersed in a beaker of water.
With pure water on both sides of the membrane, the
escaping tendency of molecules through the membrane
from either side is the same.
Now if molecules of some solute that cannot pass through
the membrane are added to the tube, but not to the beaker,
the escaping tendency of water molecules from the tube
Although the rate of flow of water molecules through
the membrane into the tube is
unimpeded, the reverse flow from tube to beaker
is hindered, since not
every molecule that approaches the membrane from the
inside surface will be a water molecule.
If only 90% of the molecules in the tube are H2O, then
the flow of water out to the beaker will be only 90%
as great. More water will flow in than out, and the
water level will rise in the tube.