CHAPTER XVIII. NITRIC ACID

85. Preparation. Experiment 52.--To 10 g. KNO3 or NaNO3, in a
flask, add 15 cc. H2SO4. Securely fasten the cork of the d.t., as
HNO3 is likely to loosen it, and pass the other end to the bottom
of a t.t. held deep in a bottle of water (Fig. 26). Apply heat,
and collect 4 or 5 cc.of the liquid. The usual reaction is: KNO3
+ H2SO4 = HKSO4 + HNO3. With greater heat, 2 KNO3 + H2SO4 = K2SO4
+ 2HNO3. Which is most economical of KNO3? Of H2SO4? Instead of a
flask, a t.t. may be used if desired (Fig. 27).

86. Properties and Tests.

Experiment 53.--(1) Note the color of the prepared liquid. (2)
Put a drop on the finger; then wash it off at once. (3) Dip a
quill or piece of white silk into it; then wash off the acid.
What color is imparted to animal substances? (4) Add a little to
a few bits of Cu turnings, or to a Cu coin. Write the equation.
(5) To 2 cc.indigo solution, add 2 cc. HNO3. State the leading
properties of HNO3, from these tests.

87. Chemically Pure HNO3 is a Colorless Liquid.-- The yellow
color of that prepared in Experiment 52 is due to liquid NO2
dissolved in it. It is then called fuming HNO3, and is very
strong. NO2 is formed at a high temperature.

Commercial or ordinary HNO3, is made from NaNO3, this being
cheaper than KNO3; it is about half water.

88. Uses. HNO3 is the basis of many nitrates, as AgNO3, used for
photography, Ba(NO3)2 and Sr(NO3)2 for fire-works, and others for
dyeing and printing calico; it is employed in making aqua regia,
sulphuric acid, nitro-glycerine, gun-cotton, aniline colors,
zylonite, etc.

Enough experiments have been performed to answer the question
whether some acids can be prepared from their salts. H2SO4 is not
so made, because no acid is strong enough to act on its salts. In
making HCl, HNO3, etc., sulphuric acid was used, being the
strongest.

AQUA REGIA.

89. Preparation and Action. Experiment 54.--Into a t.t. put 2 cc.
HNO3, and 14 qcm. of either Au leaf or Pt. Warm in a flame. If
the metal is pure, no action takes place. Into another tube put 6
cc. HCl and add a similar leaf. Heat this also. There should be
no action. Pour the contents of one t.t. into the other. Note the
effect. Which is stronger, one of the acids, or the combination
of the two? Note the odor. It is that of Cl. 3HCl + HNO3 = NOCl +
2H2O + Cl2. This reaction is approximate only. The strength is
owing to nascent chlorine, which unites with Au. Au + 3Cl =
AuCl3. If Pt be used, PtCl4 is produced. No other acid except
nitro-hydrochloric will dissolve Au or Pt; hence the ancients
called it aqua regia, or king of liquids. It must be made as
wanted, since it cannot be kept and retain its strength.