and oxygen, are
formed from it. This statement in the form of an equation is

(2) H_{2}O = 2H + O.

The coefficient before the symbol for hydrogen indicates that a single
molecule of water yields two atoms of hydrogen on decomposition.

In like manner the combination of sulphur with iron is expressed by the
equation

(3) Fe + S = FeS.

The decomposition of potassium chlorate by heat takes place as
represented by the equation

(4) KClO_{3} = KCl + 3O.

~Reading of equations.~ Since equations are simply a kind of shorthand way
of indicating chemical changes which occur under certain conditions, in
reading an equation the full statement for which it stands should be
given. Equation (1) should be read, "Mercuric oxide when heated gives
mercury and oxygen"; equation (2) is equivalent to the statement, "When
electrolyzed, water produces hydrogen and oxygen"; equation (3), "When
heated together iron and sulphur unite to form iron sulphide"; equation
(4), "Potassium chlorate when heated yields potassium chloride and
oxygen."

~Knowledge required for writing equations.~ In order to write such
equations correctly, a considerable amount of exact knowledge is
required. Thus, in equation (1) the fact that red oxide of mercury has
the composition represented by the formula HgO, that it is decomposed by
heat, that in this decomposition mercury and oxygen are formed and no
other products,--all these facts must be ascertained by exact experiment
before the equation can be written. An equation expressing these facts
will then have much value.

Having obtained an equation describing the conduct of mercuric oxide on
being heated, it will not do to assume that other oxides will behave in
like manner. Iron oxide (FeO) resembles mercuric oxide in many respects,
but it undergoes no change at all when heated. Manganese dioxide, the
black substance used in the preparation of oxygen, has the formula
MnO_{2}. When this substance is heated oxygen is set free, but the metal
manganese is not liberated; instead, a different oxide of manganese
containing less oxygen is produced. The equation representing the
reaction is

3MnO_{2} = Mn_{3}O_{4} + 2O.

~Classes of reactions.~ When a chemical change takes place in a substance
the substance is said to undergo a reaction. Although a great many
different reactions will be met in the study of chemistry, they may all
be grouped under the following heads.

1. _Addition._ This is