formed in it. It is unsatisfactory in that it
does not give full information about the way in which the
reaction takes place. For example, it does not suggest that
hydrogen is at first formed, and subsequently transformed into
water. It is always much more important to remember the steps
in a chemical reaction than to remember the equation expressing
the complete action; for if these steps in the reaction are
understood, the complete equation is easily obtained in the
manner just described.

~Salts of nitric acid,--nitrates.~ The salts of nitric acid are called
nitrates. Many of these salts will be described in the study of the
metals. They are all soluble in water, and when heated to a high
temperature undergo decomposition. In a few cases a nitrate on being
heated evolves oxygen, forming a nitrite:

NaNO_{3} = NaNO_{2} + O.

In other cases the decomposition goes further, and the metal is left as
oxide:

Cu(NO_{3})_{2} = CuO + 2NO_{2} + O.

~Nitrous acid~ (HNO_{2}). It is an easy matter to obtain sodium nitrite
(NaNO_{2}), as the reaction given on the previous page indicates.
Instead of merely heating the nitrate, it is better to heat it together
with a mild reducing agent, such as lead, when the reaction takes place
which is expressed by the equation

NaNO_{3} + Pb = PbO + NaNO_{2}.

When sodium nitrite is treated with an acid, such as sulphuric acid, it
is decomposed and nitrous acid is set free:

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

The acid is very unstable, however, and decomposes readily into water
and nitrogen trioxide (N_{2}O_{3}):

2HNO_{2} = H_{2}O + N_{2}O_{3}.

Dilute solutions of the acid, however, can be obtained.

COMPOUNDS OF NITROGEN WITH OXYGEN

Nitrogen combines with oxygen to form five different oxides. The
formulas and names of these are as follows:

N_{2}O nitrous oxide.
NO nitric oxide.
NO_{2} nitrogen peroxide.
N_{2}O_{3} nitrogen trioxide, or nitrous anhydride.
N_{2}O_{5} nitrogen pentoxide, or nitric anhydride.

These will now be briefly discussed.

~Nitrous oxide~ (_laughing gas_) (N_{2}O). Ammonium nitrate, like all
nitrates, undergoes decomposition when heated; and owing to the fact
that it contains no metal, but does contain both oxygen and hydrogen,
the reaction is a peculiar one. It is represented by the equation

NH_{4}NO_{3} = 2H_{2}O + N_{2