CHLORHYDRIC ACID.
77. We have seen that salts are made by the union of acids and
bases. Can these last be obtained from salts?
78. Preparation of HCl.
Experiment 47.--Into a flask put 10 g. coarse NaCl, and add 20
cc. H2SO4. Connect with Woulff bottles [Woulff bottles may be
made by fitting to wide-mouthed bottles corks with three holes,
through which pass two delivery tubes, and a central safety tube
dipping into the liquid, as in Figures 22 and 23.] partly filled
with water, as in Figure 22. One bottle is enough to collect the
HCl; but in that case it is less pure, since some H2SO4 and other
impurities are carried over. Several may be connected, as in
Figure 23. The water in the first bottle must be nearly saturated
before much gas will pass into the second. Heat the mixture 15 or
20 minutes, not very strongly, to prevent too much foaming.
Notice any current in the first bottle. NaCl + H2SO4 = HNaSO4 +
HCl. Intense heat would have given: 2NaCl + H2SO4 = Na2SO4 +
2HCl. Compare these equations with those for HNO3. In which
equation above is H2SO4 used most economically? Both reactions
take place when HCl is made on the large scale.
(Fig. 22)
79. Tests. Experiment 48.--(1) Test with litmus the liquid in
each Woulffbottle. (2) Put a piece of Zn into a t.t. and cover it
with liquid from the first bottle. Write the reaction, and test
the gas. (3) To 2 cc.solution AgNO3 in a t.t. add 2 cc.of the
acid. Describe, and write the reaction. Is AgCl soluble in water?
(4) Into a t.t. pour 5 cc.Pb(NO3)2 solution, and add the same
amount of prepared acid. Give the description and the reaction.
(5) In the same way test the acid with Hg2(NO3)2 solution, giving
the reaction. (6) Drake a little HCl in a t.t., and bring the gas
escaping from the d.t. in contact with a burning stick. Does it
support the combustion of C? (7) Hold a piece of dry litmus paper
against it. [figure 23] (8) Hold it over 2 cc.of NH4OH in an
evaporating-dish. Describe, name the product, and write the
reaction. (3), (4), (5), (8), are characteristic tests for this
acid.
80. Chlorhydric, Hydrochloric or Muriatic, Acid is a Gas.--As
used, it is dissolved, in water, for which it has great affinity.
Water will hold, according to temperature, from 400 to 500 times
its volume of HCl. Hundreds of thousands of tons of the acid are
annually made, mostly in Europe, as a bye-product in Na2CO3
manufacture. The gas is passed into towers through which a spray
of water falls; this absorbs it. The yellow color in most
commercial HCl indicates impurities, some of which are Fe, S, As,
and organic matter. As, S, etc., come from the pyrites used in
making H2SO4. Chemically pure (C.P.) acid is freed from these,
and is without color. The gas may be dried by passing it through
a glass tube holding CaCl2 (Fig. 16) and collecting it over
mercury.
The muriatic acid of commerce consists of about two- thirds water
by weight. HCl can also be made by direct union of its
constituents.81. Uses.--HCl is used to make Cl, and also
bleaching- powder. Its use as a reagent in the laboratory is
illustrated by the following experiment:-- Experiment 49.--Put
into a t.t. 2 cc. AgNO3 solution, add 5 cc. H2O, then add slowly
HCl so long as a ppt. (precipitate) is formed. This ppt. is AgCl.
Now in another t.t. put 2 cc. Cu(NO3)2, solution, add 5 cc. H2O,
then a little HCl. No ppt. is formed. Now if a solution of AgNO3
and a solution of Cu(NO3)2 were mixed, and HCl added, it is
evident that the silver would be precipitated as chloride of
silver, while the copper would remain in solution. If now this be
filtered, the silver will remain on the filter paper, while in
the filtrate will be the copper. Thus we shall have performed an
analysis, or separated one metal from another. Perform it. Note,
however, that any soluble chloride, as NaCl, would produce the
same result as HCl.
BROMHYDRIC AND IODIHYDRIC ACIDS.
82. NaCl, being the most abundant compound of Cl, is the source
of commercial HCl. KCl treated in the same way would give a like
product. Theoretically HBr and HI might be made in the same way
from NaBr and NaI, but the affinity of H for Br and I is weak,
and the acids separate into their elements, when thus prepared.
83. To make HI.
Experiment 50.--Drop into a t.t. three or four crystals of I, and
add 10 cc. H2O. Hold in the water the end of a d.t. from which
H2S gas is escaping. Observe any deposit, and write the reaction.
FLUORHYDRIC ACID.
84. Preparation and Action.
Experiment 51.--Put 3 or 4 g. powdered CaF2, i.e. fluor spar or
fluorite, into a shallow lead tray, e.g. 4x5 cm, and pour over it
4 or 5 cc. H2SO4. A piece of glass large enough to cover this
should previously be warmed and covered on one side with a very
thin coat of beeswax. To distribute itevenly, warm the other side
of the glass over a flame. When cool, scratch a design (Fig. 24)
through the wax with a sharp metallic point. Lay the glass, film
side down, over the lead tray. Warm this five minutes or more by
placing it high over a small flame (Fig. 25) to avoid melting the
wax. Do not inhale the fumes. Take away the lamp, and leave the
tray and glass where it is not cold, for half an hour or more.
Then remove the wax and clean the glass with naphtha or benzine.
Look for the etching.
Two things should have occurred: (1) the generation of HF. Write
the equation for it. (2) Its etching action on glass. In this
last process HF acts on SiO2 of the glass, forming H2O and SiF4.
Why cannot HF be kept in glass bottles?
A dilute solution of HF, which is a gas, may be kept in gutta
percha bottles, the anhydrous acid in platinum only; but for the
most part, it is used as soon as made, its chief use being to
etch designs on glass-ware. Glass is also often etched by a blast
of sand (SiO2).
Notice the absence of O in the acids HF, HCI, HBr, HI, and that
each is a gas. HF is the only acid that will dissolve or act
appreciably on glass.