An Elementary Study of Chemistry | Page 9

oxides formed by the
combustion of any substance are called products of combustion of that
substance. Thus oxide of sulphur is the product of the combustion of
sulphur; oxide of iron is the product of the combustion of iron. It is
evident that the products of the combustion of any substance must
weigh more than the original substance, the increase in weight
corresponding to the amount of oxygen taken up in the act of
combustion. For example, when iron burns the oxide of iron formed
weighs more than the original iron.
In some cases the products of combustion are invisible gases, so that

the substance undergoing combustion is apparently destroyed. Thus,
when a candle burns it is consumed, and so far as the eye can judge
nothing is formed during combustion. That invisible gases are formed,
however, and that the weight of these is greater than the weight of the
candle may be shown by the following experiment.
[Illustration: Fig. 7]
A lamp chimney is filled with sticks of the compound known as sodium
hydroxide (caustic soda), and suspended from the beam of the balance,
as shown in Fig. 7. A piece of candle is placed on the balance pan so
that the wick comes just below the chimney, and the balance is brought
to a level by adding weights to the other pan. The candle is then lighted.
The products formed pass up through the chimney and are absorbed by
the sodium hydroxide. Although the candle burns away, the pan upon
which it rests slowly sinks, showing that the combustion is attended by
an increase in weight.
~Combustion in air and in oxygen.~ Combustion in air and in oxygen
differs only in rapidity, the products formed being exactly the same.
That the process should take place less rapidly in the former is readily
understood, for the air is only about one fifth oxygen, the remaining
four fifths being inert gases. Not only is less oxygen available, but
much of the heat is absorbed in raising the temperature of the inert
gases surrounding the substance undergoing combustion, and the
temperature reached in the combustion is therefore less.
~Phlogiston theory of combustion.~ The French chemist Lavoisier
(1743-1794), who gave to oxygen its name was the first to show that
combustion is due to union with oxygen. Previous to his time
combustion was supposed to be due to the presence of a substance or
principle called phlogiston. One substance was thought to be more
combustible than another because it contained more phlogiston. Coal,
for example, was thought to be very rich in phlogiston. The ashes left
after combustion would not burn because all the phlogiston had
escaped. If the phlogiston could be restored in any way, the substance
would then become combustible again. Although this view seems
absurd to us in the light of our present knowledge, it formerly had

general acceptance. The discovery of oxygen led Lavoisier to
investigate the subject, and through his experiments he arrived at the
true explanation of combustion. The discovery of oxygen together with
the part it plays in combustion is generally regarded as the most
important discovery in the history of chemistry. It marked the dawn of
a new period in the growth of the science.
~Combustion in the broad sense.~ According to the definition given
above, the presence of oxygen is necessary for combustion. The term is
sometimes used, however, in a broader sense to designate any chemical
change attended by the evolution of heat and light. Thus iron and
sulphur, or hydrogen and chlorine under certain conditions, will
combine so rapidly that light is evolved, and the action is called a
combustion. Whenever combustion takes place in the air, however, the
process is one of oxidation.
~Spontaneous combustion.~ The temperature reached in a given
chemical action, such as oxidation, depends upon the rate at which the
reaction takes place. This rate is usually increased by raising the
temperature of the substances taking part in the action.
When a slow oxidation takes place under such conditions that the heat
generated is not lost by being conducted away, the temperature of the
substance undergoing oxidation is raised, and this in turn hastens the
rate of oxidation. The rise in temperature may continue in this way
until the kindling temperature of the substance is reached, when
combustion begins. Combustion occurring in this way is called
spontaneous combustion.
Certain oils, such as the linseed oil used in paints, slowly undergo
oxidation at ordinary temperatures, and not infrequently the origin of
fires has been traced to the spontaneous combustion of oily rags. The
spontaneous combustion of hay has been known to set barns on fire.
Heaps of coal have been found to be on fire when spontaneous
combustion offered the only possible explanation.
~Importance of oxygen.~ 1. Oxygen is essential