Man's physical universe

xanabras

248 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

valuable mineral substances by selective dialysis. One method of

eliminating this objection is the practice that many people have of

drinking a glass or two of lukewarm normal saline solution about a

half-hour before breakfast. The term "normal" here refers to the fact

that the saline solution, which is about 0.9-1 per cent and contains

two level teaspoons of common salt to a quart of water, is isotonic

with the blood. The body receives its food by dialysis through the

membranes in the intestinal tract, so that both dialysis and osmosis

take place at the same time. A solution having the same osmotic

pressure as the blood would be called isosmotic with the blood, but such

a solution might still change in concentration as the result of dialysis

either from it to the blood or vice versa. A solution which does not

change either by osmosis or dialysis when placed on the opposite

side of the membrane to that of the blood is said to be isotonic with the

blood.

The chemical garden is an excellent illustration of osmosis. It is

prepared by dropping little crystals of very soluble colored salts such

as ferric chloride, nickel chloride, cobalt nitrate, manganese nitrate,

uranium nitrate, copper sulfate, etc., into a 10 per cent sodium silicate

solution. These salts dissolve in the film of water surrounding the

crystal, and the resulting solution reacts with the sodium silicate to

form membranes. Water passes by osmosis into the crystal side of the

membrane and finally bursts the membrane. The solutions thus form

new membranes, and the growth continues. Your instructor will

undoubtedly demonstrate the chemical garden for you.

The lowering of the vapor pressure and the freezing-point, the elevation

of the boiling-point, and the osmotic pressure of solutions are all due to

the number of molecules present rather than to their chemical composition.

It is possible, therefore, to estimate the relative number of molecules

of any solute present in a given weight of solvent by measuring the

above properties of the solutions in question.

It should be added here that there is a group of substances for which

the above properties are abnormally high. The simplest way to explain

such abnormalities is by saying that the molecules must have subdivided

to form a larger number of smaller particles, each of which

produced the same effect as the original molecule. Such abnormalities,

characteristic of solutions of electrolytes, will be studied in a later unit.

STUDY QUESTIONS

1. Define: solvent, solute, and solution.

2. How does water behave when it is cooled from 20° C. to —10° C?

3. Give an example of a supersaturated solution.

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