eems to take
place. If, however, the mixture is heated in a retort, nitric acid is
given off as a vapor and may be easily condensed to a liquid by passing
the vapor into a tube surrounded by cold water, as shown in Fig. 37. An
examination of the liquid left in the retort shows that it contains
sodium acid sulphate (NaHSO_{4}), so that the reaction may be
represented by the equation

NaNO_{3} + H_{2}SO_{4} = NaHSO_{4} + HNO_{3}.

If a smaller quantity of sulphuric acid is taken and the
mixture is heated to a high temperature, normal sodium sulphate
is formed:

2NaNO_{3} + H_{2}SO_{4} = Na_{2}SO_{4} + 2HNO_{3}.

In this case, however, the higher temperature required
decomposes a part of the nitric acid.

~The commercial preparation of nitric acid.~ Fig. 38 illustrates
a form of apparatus used in the preparation of nitric acid on a
large scale. Sodium nitrate and sulphuric acid are heated in
the iron retort A. The resulting acid vapors pass in the
direction indicated by the arrows, and are condensed in the
glass tubes B, which are covered with cloth kept cool by
streams of water. These tubes are inclined so that the liquid
resulting from the condensation of the vapors runs back into
C and is drawn off into large vessels (D).

[Illustration Fig. 38]

~Physical properties of nitric acid.~ Pure nitric acid is a colorless
liquid, which boils at about 86 deg. and has a density of 1.56. The
concentrated acid of commerce contains about 68% of the acid, the
remainder being water. Such a mixture has a density of 1.4. The
concentrated acid fumes somewhat in moist air, and has a sharp choking
odor.

~Chemical properties.~ The most important chemical properties of nitric
acid are the following.

1. _Acid properties._ As the name indicates, this substance is an acid,
and has all the properties of that class of substances. It changes blue
litmus red and has a sour taste in dilute solutions. It forms hydrogen
ions in solution and neutralizes bases forming salts. It also acts upon
the oxides of most metals, forming a salt and water. It is one of the
strongest acids.

2. _Decomposition on heating._ When boiled, or exposed for some time to
sunlight, it suffers a partial decomposition according to the equation

2HNO_{3} = H_{2}O + 2NO_{2} + O.

The substance NO_{2}, called nitrogen peroxide, is a brownish gas, which
is readily soluble in w