Why does strong soap make your face sting?
How is soap made?
"Contains no free alkali," "Will not injure the most delicate of
fabrics," "99-44/100% pure,"--such phrases as these are used in
advertising soaps. What is meant by 99-44/100% pure? What is free
alkali? Why should any soap injure fabrics? What makes a soap
"strong"?
The answer to all these questions is that there are some substances
called _bases_, which are the opposites of acids, and some of which
are as powerful as acids. Lye, ammonia, caustic soda, and baking and
washing soda are common bases. The strong bases, like lye and caustic
soda, are also called _alkalies_. If you want to see what a strong
base--an alkali--will do to "the most delicate of fabrics," and to
fabrics that are not so delicate, for that matter, try the following
experiment:
EXPERIMENT 108. _To be done by the teacher._ If you get any
alkali on your skin or clothes, wash it off immediately with
vinegar or lemon juice.
Put half a teaspoonful of lye and a quarter of a cup of water
into a beaker, a small pan, or an evaporating dish. Bring it
to a _gentle_ boil. Drop a small piece of woolen cloth and a
small piece of silk cloth into it and let them boil gently
for a couple of minutes. What happens to them? Try a piece of
plain cotton cloth, and then a piece of cloth that is mixed
wool and cotton or mixed silk and cotton. What happens to
them? This is a very good test to determine whether any goods
you buy are pure silk or wool, or whether there is a cotton
thread mixed with them. Drop one end of a long hair into the
hot lye solution. What happens to it? Drop a speck of meat or
a piece of finger nail into it.
From this experiment you can readily see why lye will burn your skin
and ruin your clothes. You can also see how it softens the food that
sticks to the bottom of the cooking pan and makes the pan easy to
clean. Lye is one of the strongest bases or alkalies in the world.
[Illustration: FIG. 184. The lye has changed the wool cloth to a
jelly.]
HOW SOAP IS MADE. When lye and grease are boiled together, they form
soap. You cannot very well make soap in the laboratory now, as the
measurements must be exact and you need a good deal of strong lye to
make it in a quantity large enough to use. But the fact that soap is
made with oil, fat, or grease boiled with lye, or caustic soda, which
is almost the same thing, shows why a soap must be 99-44/100% pure,
or something like that, if it is not to injure "the most delicate
fabric." If a little too much lye is used there will be free alkali
in the soap, and it will make your hands harsh and sore and spoil the
clothes you are washing. A "pure" soap is one with no free alkali
in it. A "strong" soap is one that does have some free alkali in it;
there is a little too much lye for the oil or fat, so some lye is left
uncombined when the soap is made. This free alkali cleans things well,
but it injures hands and clothes.
When the drainpipe of a kitchen sink is stopped up, you can often
clear it by sprinkling lye down it, and then adding boiling water.
_If you ever do this, stand well back so that no lye will spatter into
your face; it sputters when the boiling water strikes it._ The grease
in the drainpipe combines with the lye when the hot water comes
down; then the soap that is formed is carried down the pipe, partly
dissolved by the hot water.
When you sponge a grease spot with ammonia, the same sort of chemical
action takes place. The ammonia is a base; it combines with the grease
to form soap, and this soap rinses out of the cloth.
THE LITMUS TEST. To tell what things are bases and what are acids,
a piece of paper dyed with litmus is ordinarily used. Litmus is made
from a plant (lichen). This paper is called _litmus paper_. Try the
following experiment with litmus paper:
EXPERIMENT 109. Pour a few drops of ammonia, a base, into a
cup. Into another cup pour a few drops of vinegar, an acid.
Dip your litmus paper first into one, then into the other, and
then back into the first. What color does the vinegar turn
it? the ammonia? Try lemon juice; diluted hydrochloric acid; a
_very_ dilute lye solution.
This is called the _litmus test_. All ordinary acids, if not too
strong, will turn litmus pink. All bases or alkalies will turn it
blue. If it is already pink when you put it into an acid, it will stay
pink, of course; if it is already blue when you put it into a base, it
will stay blue. But if you put a piece of litmus paper into something
that is neither an acid nor a base, like sugar or salt, it will still
stay the same color. So, to test for a base, use a piece of litmus
paper that is pink and see if it turns blue, or if you want to test
for an acid, use blue litmus paper. Do this experiment:
EXPERIMENT 110. With pink and blue litmus paper, test the
different substances named below to see which are acids and
which are bases. Make a list of all the acids and another list
for all the bases. Do not put down anything that is neither
acid or base. You cannot be sure a thing is an acid unless it
turns _blue_ litmus _pink_. A piece of pink litmus would stay
pink in an acid, but it would also stay pink in things that
were neither acid nor base, like salt or water. In the same
way you cannot be sure a thing is a base unless it turns
_pink_ litmus _blue_. Here is a list of things to try: 1,
sugar; 2, orange; 3, dilute sulfuric acid; 4, baking soda in
water; 5, alum in water; 6, washing soda in water; 7, ammonia;
8, dilute lye; 9, lemon juice; 10, vinegar; 11, washing powder
in water; 12, sour milk; 13, cornstarch in water; 14, wet
kitchen soap; 15, oil; 16, salt in water.
You may have to make the orange and sour milk test at home. You may
take two pieces of litmus paper home with you and test anything else
that you may care to. If you have a garden, try the soil in it. If it
is acid it needs lime.
_APPLICATION 81._ A boy spilled some greasy soup on his best
dark blue coat. Which of the following methods would have
served to clean the coat? to sponge it (a) with cold water;
(b) with water (hot) and ammonia; (c) with hot water and
vinegar; (d) with concentrated nitric acid; to sprinkle lye on
the spot and pour boiling water over it.
_APPLICATION 82._ A woman scorched the oatmeal she was cooking
for breakfast. When she wanted to wash the pan, she found that
the blackened cereal stuck fast to the bottom. Which of the
following things would have served best to loosen the burned
oatmeal from the pan: lye and hot water, ammonia, vinegar,
salt water, lemon juice?
INFERENCE EXERCISE
Explain the following:
521. After clothes have been washed with washing soda or
strong soap, they should be thoroughly rinsed. Otherwise they
will be badly eaten as they dry.
522. Carbon will burn; oxygen will support combustion; yet
carbon dioxid (CO_2), which is made of both these elements,
will neither burn nor support combustion.
523. You can clean silver by putting it in hot soda solution
in contact with aluminum.
524. When you stub your toe while walking, you tend to fall
forward.
525. Electric lamps glow when you turn on the switch.
526. If you use much ammonia in washing clothes or cleaning,
your hands become harsh and dry.
527. If a person swallows lye or caustic soda, he should
immediately drink as much vegetable oil or animal oil as
possible.
528. Water is made of hydrogen and oxygen; air is made of
nitrogen and oxygen; yet while things will not burn in water,
they will burn easily in air.
529. The backs of books that have been kept in cases for
several years are not as bright colored as the side covers.
530. If you try to burn a book or magazine in a grate, only
the outer pages and edges burn.