Man's physical universe

xanabras

REGULARITIES OF SOLUTIONS 247

tant to note that the laws dealing with the behavior of substances in

solution apply only to very dilute solutions.

Osmosis is explained in a number of different ways.

The sieve theory,

for example, assumes that the solute particles are too large to pass

through the openings in the membrane, while the solvent molecules

are sufficiently small to do so. There is considerable evidence that

each solute particle is joined to one or more solvent particles in solution.

This fits into the sieve theory nicely and also explains why the

gas laws do not apply to concentrated solutions. In such solutions so

much of the solvent would thus be joined to the solute particles that

there would be a much smaller volume of free solvent than there is

assumed to be in making calculations involving the volume.

Osmosis is also explained as the result of the solution of one of the

phases in the membrane.

If particles the size of solvated ^ solute particles could pass through

a membrane whose openings were too small to admit the passage of

the still

larger colloidal particles, the solute and colloid would thus be

separated. Of course, openings large enough to admit the passage of

solute particles would also admit the solvent molecules, so osmosis

always takes place during dialysis. Dialysis does not always take

place, however, when there is osmosis.

Osmosis and dialysis have many important applications in living

organisms because every living cell is surrounded by a membrane

through which it makes necessary interchanges with its environment.

If oysters are placed in distilled water they swell because of osmosis,

inasmuch as the solutions inside the oyster have a much higher osmotic

pressure than distilled water — they have been in equilibrium with

the sea water in which the oyster grew. Some unscrupulous markets

have taken advantage of this idea to increase the size and weight of

their oysters before selling them.

If plant cells are placed in salt water, they wilt and lose their turgor.

This is due to the fact that water passes from the plant cells to the

salt water. Such a process is called plasmolysis.

One type of remedy for intestinal stasis involves the use of saline

laxatives. Such substances as sodium sulfate (Glauber's salt), magnesium

sulfate (Epsom salts), magnesium citrate, and milk of magnesia

flush out the intestinal tract by the water drawn from the blood as a

result of the higher osmotic pressure their presence produces in the

intestines.

One objection to the use of saline laxatives is that they not only

remove water from the blood but that they also bring about a loss of

• When a solute combines with a portion of the solvent, the solute is solvated.

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