Man's physical universe

xanabras

246 PHYSICAL LIMITATIONS HAVE BEEN OVERCOME

Other things being equal, the substance having the smallest molecular

weight would be selected for use as an antifreeze substance.

Other important considerations are cost and the volatility of the antifreeze

substance. Inasmuch as ethyl and methyl alcohols boil below

the temperature of water, they are gradually lost when the automobile

engine gets hot and would have to be renewed two or three times during

a winter. The other substances would not be lost in this way because

they are less volatile than water, and it might be more economical to

use one filling of such a substance even at a higher price than several

fillings of the cheaper, more volatile substances.

Salt is mixed with ice in making ice cream in order to lower the

melting-point of the ice.

This is necessary because the sugar and other

solutes dissolved in the water of the ice cream lower its freezing-point

below the melting-point of ice. Salt is scattered on icy walks to remove

the ice because it lowers the melting-point of the ice and thus causes

it

to melt.

Ocean water does not freeze as readily as fresh water because of the

salts dissolved in it.

Brine is used in commercial ice plants to conduct to the cooling-coils

the heat from the cans of water to be frozen.

4. The Osmotic Pressure Is Produced. If a membrane, such as

parchment paper or an animal bladder, is placed between two solutions

of different concentrations, it will be found that solvent particles will

pass through the membrane in the direction of the more concentrated

solution more rapidly than the solute particles will

pass through the

membrane in the opposite direction. The passage of solvent through

a membrane into a solution is called osmosis; the passage of a solute

through a membrane is called dialysis.

Most membranes permit both

dialysis and osmosis; but if a proper membrane could be secured,

osmosis alone would take place.

If osmosis were allowed to continue until the water ceased to rise

in the tube, the hydrostatic pressure thus produced would be a measure

of the osmotic pressure of the solution.

It has been found that the osmotic

pressure of dilute solutions of equimolecular concentration is the satne for

different substances when there is no dissociation of the solute and that

it is independent of the character of the solute.

The osmotic pressure exerted by a substance in solution is equal to

the gas pressure which that substance would exert if it were a gas

occupying the same volume as that of the solvent, under the same

conditions of temperature. The solution statie is very similar to the

gaseous state in that osmotic pressure is

affected in the same way by

It is impor-

temperature and volume changes as are gaseous pressures.

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