When you put soda in vinegar, what makes the vinegar less
sour?
When we use sour milk for cooking, why does the food not taste
sour?
One of the most interesting and important facts about acids and bases
is that if they are put together in the right proportions they turn
to salt and water. Strong hydrochloric acid (HCl), for instance, will
attack the skin and clothes, as you know; if you should drink it, it
would kill you. Caustic soda (NaOH), a kind of lye, is such a strong
alkali that it would dissolve the skin of your mouth in the way
that lye dissolved hair in Experiment 108. Yet if you put these two
strongly poisonous chemicals together, they promptly turn to ordinary
table salt (NaCl) and water (H_2O). Or, as the chemists write it:
NaOH+HCl -> NaCl+H_2O.
You can make this happen yourself in the following experiment, using
the acid and base dilute enough so that they will not hurt you:
EXPERIMENT 111. Although strong hydrochloric acid and strong
caustic soda are dangerous, if they are diluted with enough
water they are perfectly harmless. You will find two bottles,
one labeled "_caustic soda_ (NaOH) diluted for tasting,"
and the other labeled "_hydrochloric acid_ (HCl) diluted
for tasting." From one bottle take a little in the medicine
dropper and let a drop fall on your tongue. Taste the contents
of the other bottle in the same way. _It is not usually safe
to taste things in the laboratory. Taste only those things
which are marked "for tasting."_
Now put a teaspoonful of the same hydrochloric acid into a
clean evaporating dish. Lay a piece of litmus paper in the
bottom of the dish. With a medicine dropper gradually add the
dilute caustic soda (NaOH), stirring as you add it. Watch the
litmus paper. When the litmus paper begins to turn blue, add
the dilute caustic soda drop by drop until the litmus paper
stays blue when you stir the mixture. Now add a drop or two
more of the acid until the litmus turns pink again. Taste the
mixture.
Put the evaporating dish on the wire gauze over a Bunsen
burner, and bring the liquid to a boil. Boil it gently until
it begins to sputter. Then take the Bunsen burner in your hand
and hold it under the dish for a couple of seconds; remove it
for a few seconds, and then again hold it under the dish for a
couple of seconds; remove it once more, and keep this up until
the water has all evaporated and left dry white crystals and
powder in the bottom of the dish. As soon as the dish is cool,
taste the crystals and powder. What are they?
Is salt an acid or a base?
Whenever you put acids and bases together, you get some kind of salt
and water. Thus the chlorine (Cl) of the hydrochloric acid (HCl)
combines with the sodium (Na) of caustic soda (NaOH) to form ordinary
table salt, sodium chloride (NaCl), while the hydrogen (H) of the
hydrochloric acid (HCl) combines with the oxygen and hydrogen (OH) of
the caustic soda (NaOH) to form water (H_2O). Chemists write this as
follows:
NaOH+HCl -> NaCl+H_2O.
WHY SOUR MILK PANCAKES ARE NOT SOUR. It is because bases neutralize
acids that you put baking soda with sour milk when you make sour milk
pancakes or muffins. The soda is a weak base. The sour milk is a weak
acid. The soda neutralizes the acid, changing it into a kind of salt
and plain water. Therefore the sour milk pancakes or muffins do not
taste sour.
In the same way a little soda keeps tomatoes from curdling the milk
when it is added to make cream of tomato soup. It is the acid in the
tomatoes that curdles milk. If you neutralize the acid by adding a
base, there is no acid left to curdle the milk; the acid and base turn
to water and a kind of salt.
When you did an experiment with strong acid, you were advised to have
some ammonia at hand to wash off any acid that might get on your skin
or clothes. The ammonia, being a base, would immediately neutralize
the acid and therefore keep it from doing any damage. Lye also would
neutralize the acid, but if you used the least bit too much, the lye
would do as much harm as the acid. That is why you should use a weak
base, like ammonia or baking soda or washing soda, to neutralize any
acid that spills on you. Then if you get too much on, it will not do
any harm.
In the same way you were warned to have vinegar near at hand while you
worked with lye. Strong nitric acid also would neutralize the lye, but
if you happened to use a drop too much, the acid would be worse than
the lye. Vinegar, of course, would not hurt you, no matter how much
you put on.
_Any_ acid will neutralize _any_ base. But it would take a great deal
of a weak acid to neutralize a strong base or alkali; you would have
to use a great deal of vinegar to neutralize concentrated lye. In the
same way it would take a great deal of a weak base to neutralize a
strong acid; you would have to use a large amount of baking soda or
ammonia to neutralize concentrated nitric acid.
_APPLICATION 83._ A woman was cleaning kettles with lye. Her
little boy was playing near, and some lye splashed on his
hand. She looked swiftly around and saw the following things:
soap, oil, lemon, flour, peroxide, ammonia, iodine, baking
soda, essence of peppermint. Which should she have put on the
boy's hand?
_APPLICATION 84._ A teacher spilled some nitric acid on her
apron. On the shelf there were: hydrochloric acid, vinegar,
lye, caustic soda, baking soda, ammonia, salt, alcohol,
kerosene, salad oil. Which should she have put on her apron?
_APPLICATION 85._ A boy had "sour stomach." His sister said,
"Chew some gum." His aunt said, "Drink hot water with a little
peppermint in it." His mother told him to take a little baking
soda in water. His brother said, "Try some hot lemonade."
Which advice should he have followed?
_APPLICATION 86._ Two women were bleaching a faded pair of
curtains. The Javelle water which they had used was made of
bleaching powder and washing soda. Before hanging the curtains
out to dry, one of them said that she was afraid the Javelle
water would become so strong as the water evaporated from the
curtains that it would eat the curtains. They decided they had
better rinse them out with something that would counteract
the soda and lime in the Javelle water, and in the laundry and
pantry they found: ammonia, blueing, starch, washing powder,
soap, vinegar, and gasoline. Which of them, if any, would it
have been well to put in the rinsing water?
INFERENCE EXERCISE
Explain the following:
531. Solid pieces of washing soda disappear in hot water.
532. Greasy clothes put into hot water with washing soda
become clean.
533. If you hang these clothes up to dry without rinsing them,
the soda will weaken the cloth.
534. Lemon juice in the rinsing water will prevent washing
soda from injuring the clothes.
535. If you hang them in the sun, the color will fade.
536. A piece of soot blown against them will stick.
537. A drop of oil that may spatter against them will spread.
538. The clothes will be easier to iron if dampened.
539. The creases made in ironing the clothes will reappear
even if you flatten the creases out with your hand.
540. After they have been worn, washed, and ironed a number of
times, clothes are thinner than they were when they were new.