Man's physical universe

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728 MAN'S PHYSICAL WELFARE

There Are Two Kinds of Colloids.

Many colloids are relatively unstable and may be precipitated by

slight traces of acids, bases, or salts (electrolytes), whose charged ions

neutralize the charge on the colloidal particles. Such colloids are called

lyophobic (suspending-medium hating) colloids, siispensoids, or sols.

Their properties do not greatly differ from those of the suspending

medium. Thus for example, their viscosity is but little greater than

that of the suspending medium.

The precipitation of suspensoids is well illustrated by the precipitation

of the suspended particles of mud- by the ions of salt in sea

water. Deltas are built by rivers where the mud is precipitated as

the river water mixes into the salty ocean water.

Many colloids, in contrast to the suspensoids, are not easily precipitated

by electrolytes but rather show a marked attraction for the

suspending medium. They are therefore called lyophilic (suspendingmedium

loving) colloids or emulsoids. Such colloids depend upon pro-

Examples

tective films rather than electrical charges for their stability.

of such colloids are egg albumen, casein, gelatine, and fruit pectin

(the essential ingredient of "Certo").

In the preparation of mayonnaise, the egg albumen is the emulsifying

agent that keeps the oil from separating from the water. The

casein in milk prevents the butter fat from separating. Gelatine in ice

cream prevents precipitation of milk sugar and formation of ice crystals.

The tannic acid in straw protects the clay from precipitating in

brick-making.

Emulsoid Colloids Are Intimately Connected with Life Processes.

The swelling of dried fruits or seeds when placed in water is

similar

to the swelling of gelatine in water; it is due to the absorption of water

by the colloid, which has a positive affinity for the water. A sponge is

another example of a colloid which has considerable affinity for water.

This absorption of water depends upon several factors, such as the

acidity or alkalinity of the solution and the temperature. The swelling

of the tissues of the body during acidosis is an illustration of this effect.

If sufficient water is absorbed, the colloid will form a liquid or sol

form. Sometimes sols, as in the case of gelatine, will change to a gel

form on cooling.

Colloids Are Prepared by the Growth of Smaller Particles or the Division

of Larger Particles.

Inasmuch as colloidal particles range in size from the smallest,

which is about one hundred times the diameter of the hydrogen atom.

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