The osmotic pressure of a solution

solute particles and is independent of their nature. The osmotic pressure of a solution

can be measured by determining the amount of counterpressure needed to prevent osmosis;

this pressure can be very large. The osmotic pressure of a solution containing

1 mol of solute particles in 1 kg of water is about 22.4 atm, which is about the same as

the pressure exerted by 1 mol of a gas confined in a volume of 1 L at 0°C.

Osmosis has a role in many biological processes, and semipermeable membranes

occur commonly in living organisms. An example is the roots of plants, which are covered

with tiny structures called root hairs; soil water enters the plant by osmosis, passing

through the semipermeable membranes covering the root hairs. Artificial or

synthetic membranes can also be made.

Osmosis can be demonstrated with the simple laboratory setup shown in

Figure 14.9. As a result of osmotic pressure, water passes through the cellophane

membrane into the thistle tube, causing the solution level to rise. In osmosis, the net

transfer of water is always from a less concentrated to a more concentrated solution;

that is, the effect is toward equalization of the concentration on both sides of the

membrane. Note that the effective movement of water in osmosis is always from the

region of higher water concentration to the region of lower water concentration.

Osmosis can be explained by assuming that a semipermeable membrane has passages

that permit water molecules and other small molecules to pass in either direction.

Both sides of the membrane are constantly being struck by water molecules in

random motion. The number of water molecules crossing the membrane is proportional

to the number of water molecule-to-membrane impacts per unit of time. Because

the solute molecules or ions reduce the concentration of water, there are more

water molecules and thus more water molecule impacts on the side with the lower

solute concentration (more dilute solution). The greater number of water moleculeto-membrane

impacts on the dilute side thus causes a net transfer of water to the

more concentrated solution. Again, note that the overall process involves the net transfer,

by diffusion through the membrane, of water molecules from a region of higher

water concentration (dilute solution) to one of lower water concentration (more concentrated

solution).

This is a simplified picture of osmosis. No one has ever seen the hypothetical passages

that allow water molecules and other small molecules or ions to pass through

them. Alternative explanations have been proposed, but our discussion has been confined

to water solutions. Osmotic pressure is a gsolute particles and is independent of their nature. The osmotic pressure of a solution

can be measured by determining the amount of counterpressure needed to prevent osmosis;

this pressure can be very large. The osmotic pressure of a solution containing

1 mol of solute particles in 1 kg of water is about 22.4 atm, which is about the same as

the pressure exerted by 1 mol of a gas confined in a volume of 1 L at 0°C.

Osmosis has a role in many biological processes, and semipermeable membranes

occur commonly in living organisms. An example is the roots of plants, which are covered

with tiny structures called root hairs; soil water enters the plant by osmosis, passing

through the semipermeable membranes covering the root hairs. Artificial or

synthetic membranes can also be made.

Osmosis can be demonstrated with the simple laboratory setup shown in

Figure 14.9. As a result of osmotic pressure, water passes through the cellophane

membrane into the thistle tube, causing the solution level to rise. In osmosis, the net

transfer of water is always from a less concentrated to a more concentrated solution;

that is, the effect is toward equalization of the concentration on both sides of the

membrane. Note that the effective movement of water in osmosis is always from the

region of higher water concentration to the region of lower water concentration.

Osmosis can be explained by assuming that a semipermeable membrane has passages

that permit water molecules and other small molecules to pass in either direction.

Both sides of the membrane are constantly being struck by water molecules in

random motion. The number of water molecules crossing the membrane is proportional

to the number of water molecule-to-membrane impacts per unit of time. Because

the solute molecules or ions reduce the concentration of water, there are more

water molecules and thus more water molecule impacts on the side with the lower

solute concentration (more dilute solution). The greater number of water moleculeto-membrane

impacts on the dilute side thus causes a net transfer of water to the

more concentrated solution. Again, note that the

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