An Elementary Study of Chemistry | Page 7

It was given to the
element by the French chemist Lavoisier, since he believed that all
acids owe their characteristic properties to the presence of oxygen. This
view we now know to be incorrect.
~Occurrence.~ Oxygen is by far the most abundant of all the elements.
It occurs both in the free and in the combined state. In the free state it
occurs in the air, 100 volumes of dry air containing about 21 volumes
of oxygen. In the combined state it forms eight ninths of water and
nearly one half of the rocks composing the earth's crust. It is also an
important constituent of the compounds which compose plant and
animal tissues; for example, about 66% by weight of the human body is
oxygen.
~Preparation.~ Although oxygen occurs in the free state in the
atmosphere, its separation from the nitrogen and other gases with
which it is mixed is such a difficult matter that in the laboratory it has
been found more convenient to prepare it from its compounds. The
most important of the laboratory methods are the following:
1. Preparation from water. Water is a compound, consisting of 11.18%
hydrogen and 88.82% oxygen. It is easily separated into these
constituents by passing an electric current through it under suitable
conditions. The process will be described in the chapter on water.
While this method of preparation is a simple one, it is not economical.
2. Preparation from mercuric oxide. This method is of interest, since it
is the one which led to the discovery of oxygen. The oxide, which
consists of 7.4% oxygen and 92.6% mercury, is placed in a small, glass
test tube and heated. The compound is in this way decomposed into
mercury which collects on the sides of the glass tube, forming a silvery
mirror, and oxygen which, being a gas, escapes from the tube. The
presence of the oxygen is shown by lighting the end of a splint,
extinguishing the flame and bringing the glowing coal into the mouth

of the tube. The oxygen causes the glowing coal to burst into a flame.
In a similar way oxygen may be obtained from its compounds with
some of the other elements. Thus manganese dioxide, a black
compound of manganese and oxygen, when heated to about 700°, loses
one third of its oxygen, while barium dioxide, when heated, loses one
half of its oxygen.
3. Preparation from potassium chlorate (usual laboratory method).
Potassium chlorate is a white solid which consists of 31.9% potassium,
28.9% chlorine, and 39.2% oxygen. When heated it undergoes a series
of changes in which all the oxygen is finally set free, leaving a
compound of potassium and chlorine called potassium chloride. The
change may be represented as follows:
/potassium\ | | (potassium / potassium \ (potassium { chlorine } = { } +
oxygen | | chlorate) \ chlorine / chloride) \oxygen /
[Illustration: JOSEPH PRIESTLEY (English) (1733-1804)
School-teacher, theologian, philosopher, scientist; friend of Benjamin
Franklin; discoverer of oxygen; defender of the phlogiston theory; the
first to use mercury in a pneumatic trough, by which means he first
isolated in gaseous form hydrochloric acid, sulphur dioxide, and
ammonia]
The evolution of the oxygen begins at about 400°. It has been found,
however, that if the potassium chlorate is mixed with about one fourth
its weight of manganese dioxide, the oxygen is given off at a much
lower temperature. Just how the manganese dioxide brings about this
result is not definitely known. The amount of oxygen obtained from a
given weight of potassium chlorate is exactly the same whether the
manganese dioxide is present or not. So far as can be detected the
manganese dioxide undergoes no change.
[Illustration: Fig. 4]
~Directions for preparing oxygen.~ The manner of preparing oxygen

from potassium chlorate is illustrated in the accompanying diagram
(Fig. 4). A mixture consisting of one part of manganese dioxide and
four parts of potassium chlorate is placed in the flask A and gently
heated. The oxygen is evolved and escapes through the tube B. It is
collected by bringing over the end of the tube the mouth of a bottle
completely filled with water and inverted in a vessel of water, as shown
in the figure. The gas rises in the bottle and displaces the water. In the
preparation of large quantities of oxygen, a copper retort (Fig. 5) is
often substituted for the glass flask.
[Illustration: Fig. 5]
In the preparation of oxygen from potassium chlorate and manganese
dioxide, the materials used must be pure, otherwise a violent explosion
may occur. The purity of the materials is tested by heating a small
amount of the mixture in a test tube.
~The collection of gases.~ The method used for collecting oxygen
illustrates the general method used for collecting such gases as are
insoluble in water