hts of
the atoms represented by the symbols. This is always understood to be
the case in equations where the heat of reaction is given.

~Conditions of a chemical action are not indicated by equations.~
Equations do not tell the conditions under which a reaction will take
place. The equation

HgO = Hg + O

does not tell us that it is necessary to keep the mercuric oxide at a
high temperature in order that the decomposition may go on. The equation

Zn + 2HCl = ZnCl_{2} + 2H

in no way indicates the fact that the hydrochloric acid must be
dissolved in water before it will act upon the zinc. From the equation

H + Cl = HCl

it would not be suspected that the two gases hydrogen and chlorine will
unite instantly in the sunlight, but will stand mixed in the dark a long
time without change. It will therefore be necessary to pay much
attention to the details of the conditions under which a given reaction
occurs, as well as to the expression of the reaction in the form of an
equation.

EXERCISES

1. Calculate the percentage composition of the following substances:
(a) mercuric oxide; (b) potassium chlorate; (c) hydrochloric acid;
(d) sulphuric acid. Compare the results obtained with the compositions
as given in Chapters II and III.

2. Determine the percentage of copper, sulphur, oxygen, and water in
copper sulphate crystals. What weight of water can be obtained from 150
g. of this substance?

3. What weight of zinc can be dissolved in 10 g. of sulphuric acid? How
much zinc sulphate will be formed?

4. How many liters of hydrogen measured under standard conditions can be
obtained from the action of 8 g. of iron on 10 g. of sulphuric acid? How
much iron sulphate (FeSO_{4}) will be formed?

5. 10 g. of zinc were used in the preparation of hydrogen; what weight
of iron will be required to prepare an equal volume?

6. How many grams of barium dioxide will be required to prepare 1 kg. of
common hydrogen dioxide solution? What weight of barium sulphate will be
formed at the same time?

7. What weight of the compound Mn_{3}O_{4} will be formed by strongly
heating 25 g. of manganese dioxide? What volume of oxygen will be given
off at the same time, measured under standard conditions?

8. (a) What is the weight of 100 l. of hydrogen measured in a
laboratory in which the temperature is 20 deg. and pressure 750 mm.? (b)
What weight of sulphuric acid is necessary to prepare this amount of
hydrogen? (c) Th