An Elementary Study of Chemistry | Page 8

or nearly so. The vessel C (Fig. 4), containing the
water in which the bottles are inverted, is called a pneumatic trough.
~Commercial methods of preparation.~ Oxygen can now be purchased
stored under great pressure in strong steel cylinders (Fig. 6). It is
prepared either by heating a mixture of potassium chlorate and
manganese dioxide, or by separating it from the nitrogen and other
gases with which it is mixed in the atmosphere. The methods employed
for effecting this separation will be described in subsequent chapters.
[Illustration: Fig. 6]
~Physical properties.~ Oxygen is a colorless, odorless, tasteless gas,
slightly heavier than air. One liter of it, measured at a temperature of 0°
and under a pressure of one atmosphere, weighs 1.4285 g., while under
similar conditions one liter of air weighs 1.2923 g. It is but slightly
soluble in water. Oxygen, like other gases, may be liquefied by
applying very great pressure to the highly cooled gas. When the
pressure is removed the liquid oxygen passes again into the gaseous

state, since its boiling point under ordinary atmospheric pressure is
-182.5°.
~Chemical properties.~ At ordinary temperatures oxygen is not very
active chemically. Most substances are either not at all affected by it, or
the action is so slow as to escape notice. At higher temperatures,
however, it is very active, and unites directly with most of the elements.
This activity may be shown by heating various substances until just
ignited and then bringing them into vessels of the gas, when they will
burn with great brilliancy. Thus a glowing splint introduced into a jar
of oxygen bursts into flame. Sulphur burns in the air with a very weak
flame and feeble light; in oxygen, however, the flame is increased in
size and brightness. Substances which readily burn in air, such as
phosphorus, burn in oxygen with dazzling brilliancy. Even substances
which burn in air with great difficulty, such as iron, readily burn in
oxygen.
The burning of a substance in oxygen is due to the rapid combination
of the substance or of the elements composing it with the oxygen. Thus,
when sulphur burns both the oxygen and sulphur disappear as such and
there is formed a compound of the two, which is an invisible gas,
having the characteristic odor of burning sulphur. Similarly,
phosphorus on burning forms a white solid compound of phosphorus
and oxygen, while iron forms a reddish-black compound of iron and
oxygen.
~Oxidation.~ The term oxidation is applied to the chemical change
which takes place when a substance, or one of its constituent parts,
combines with oxygen. This process may take place rapidly, as in the
burning of phosphorus, or slowly, as in the oxidation (or rusting) of
iron when exposed to the air. It is always accompanied by the liberation
of heat. The amount of heat liberated by the oxidation of a definite
weight of any given substance is always the same, being entirely
independent of the rapidity of the process. If the oxidation takes place
slowly, the heat is generated so slowly that it is difficult to detect it. If
the oxidation takes place rapidly, however, the heat is generated in such
a short interval of time that the substance may become white hot or

burst into a flame.
~Combustion; kindling temperature.~ When oxidation takes place so
rapidly that the heat generated is sufficient to cause the substance to
glow or burst into a flame the process is called combustion. In order
that any substance may undergo combustion, it is necessary that it
should be heated to a certain temperature, known as the kindling
temperature. This temperature varies widely for different bodies, but is
always definite for the same body. Thus the kindling temperature of
phosphorus is far lower than that of iron, but is definite for each. When
any portion of a substance is heated until it begins to burn the
combustion will continue without the further application of heat,
provided the heat generated by the process is sufficient to bring other
parts of the substance to the kindling temperature. On the other hand, if
the heat generated is not sufficient to maintain the kindling temperature,
combustion ceases.
~Oxides.~ The compounds formed by the oxidation of any element are
called oxides. Thus in the combustion of sulphur, phosphorus, and iron,
the compounds formed are called respectively oxide of sulphur, oxide
of phosphorus, and oxide of iron. In general, then, an oxide is a
compound of oxygen with another element. A great many substances of
this class are known; in fact, the oxides of all the common elements
have been prepared, with the exception of those of fluorine and
bromine. Some of these are familiar compounds. Water, for example, is
an oxide of hydrogen, and lime an oxide of the metal calcium.
~Products of combustion.~ The particular