Man's physical universe



ashes with water. Little improvement came to soap-making until

the chemistry of the process became understood.

It is now known that soap is a metallic salt of certain organic acids,

Fats and oils are compounds

such as stearic, palmitic, and oleic acids.

of these acids with glycerine (glycerine esters) ; and, when sodium or

potassium hydroxides (caustic soda or caustic potash, NaOH or KOH)

react with fats, glycerine is liberated, and the soap is produced. The

sodium and potassium soaps are the only important ones that are

soluble in water, and for that reason these soaps alone are used for

cleaning purposes. The insoluble soaps, consisting of compounds of

these acids with other metals, such as calcium or magnesium, are used

to prepare lubricating greases, because they enable water to be emulsified

in mineral oil to form the relatively stable lubricants which we

call greases.

The principal value of the soluble soaps is that they enable fats

and greases to be emulsified by water. When the grease is removed,

the dirt which clings to it is removed also.

It is thought that soaps are

capable of emulsifying oils because one end of the soap molecule is

soluble in oils while the other end is soluble in water. Molecules of

soap are then thought to form in a layer constituting a film surrounding

each oil particle, in which all of the soap molecules are oriented with

their water-soluble ends pointing outward.

Water-softening Is an Important Problem Today.

W^hen soap is used with some waters, it forms insoluble curds and

fails to act as a detergent. Such waters are called hard. Soft water

does not produce these curds, which are insoluble soaps usually of

calcium, magnesium, or iron.

There are two kinds of hard w^ater, temporary and permanent.

Temporary hardness is caused by soluble bicarbonates of calcium,

magnesium, or iron, w'hich may be changed into insoluble carbonates

and thus precipitated by boiling. Permanent hardness is caused by

other soluble salts of these metals, such as chlorides or sulfates. It

may be removed by use of soaps; but soap is too expensive to use for

softening water economically, and the curds produced are undesirable.

Permanent hardness may also be removed by precipitation with

sodium carbonate and lime. Some cities in hard-water regions find

that it is economical to soften the whole domestic water supply by

treatment with chemicals.

Many industries require an abundant supply of soft water, and the

location of some industries is determined largdy on this basis. Hard

water not only interferes in textile, dye, and various other industries

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