be
made by heating certain nitrates, such as lead nitrate:

Pb(NO_{3})_{2} = PbO + 2NO_{2} + O.

It is a reddish-yellow gas of unpleasant odor, which is quite poisonous
when inhaled. It is heavier than air and is easily condensed to a
liquid. It dissolves in water, but this solution is not a mere physical
solution; the nitrogen peroxide is decomposed, forming a mixture of
nitric and nitrous acids:

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

Nitrogen peroxide will not combine with more oxygen; it will, however,
give up a part of its oxygen to burning substances, acting as an
oxidizing agent:

NO_{2} = NO + O.

~Acid anhydrides.~ The oxides N_{2}O_{3} (nitrogen trioxide) and
N_{2}O_{5} (nitrogen pentoxide) are rarely prepared and need not be
separately described. They bear a very interesting relation to the acids
of nitrogen. When dissolved in water they combine with the water,
forming acids:

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

N_{2}O_{5} + H_{2}O = 2HNO_{3}.

On the other hand, nitrous acid very easily decomposes, yielding water
and nitrogen trioxide, and by suitable means nitric acid likewise may be
decomposed into water and nitrogen pentoxide:

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

2HNO_{3} = H_{2}O + N_{2}O_{5}.

In view of the close relation between these oxides and the corresponding
acids, they are called _anhydrides_ of the acids, N_{2}O_{3} being
nitrous anhydride and N_{2}O_{5} nitric anhydride.

DEFINITION: _Any oxide which will combine with water to form an acid, or
which together with water is formed by the decomposition of an acid, is
called an anhydride of that acid._

EXERCISES

1. Perfectly dry ammonia does not affect litmus paper. Explain.

2. Can ammonia be dried by passing the gas through concentrated
sulphuric acid? Explain.

3. Ammonium hydroxide is a weak base, i.e. it is not highly dissociated.
When it is neutralized by strong acids the heat of reaction is less than
when strong bases are so neutralized. Suggest some possible cause for
this.

4. Why is brine used in the manufacture of artificial ice?

5. Discuss the energy changes which take place in the manufacture of
artificial ice.

6. What weight of ammonium chloride is necessary to furnish enough
ammonia to saturate 1 l. of water at 0 deg. and 760 mm.?

7. What weight of sodium nitrate is necessary to prepare 100 cc. of
commercial nitric acid? What weight of potassium nitrate is necessary to
furni