By PRAGYANSHARMA POLAVARAPU (Vrs.Somanchi)
WHAT IS FIRE?
What we generally understand by the
word combustion is burning of substances like firewood or a
heap of tree branches or even a piece of oil- soaked cloth in air . This
combustion which we see in ordinary world occurs always in plenty of air
which is all around us . Scientists discovered only in 1774 that only 21% of
air is oxygen gas which supports combustion . The rest is an inert inactive gas
called nitrogen.
From view point of science,
combustion (or burning in the plenty of air existing all around us) is a
chemical reaction taking place before our eyes at a very high
temperature of around 1000 degrees centigrade. It is a chemical
reaction at explosive speed between hot gases emitting
light in the form of flame which is the visible region of
chemical reaction between gases. But speaking from viewpoint of science it is
a chemical reaction. Everywhere in this essay we mention flame
temperature as “1000 degrees centigrade” or “around 1000 degrees centigrade” to
give a picture of the very high temperature created in the area of chemical
reaction in the burning gases.
Let us examine “combustion” ,in a
small camp fire or in a stove in a rural kitchen where dry pieces of fire wood
burn with a flame. After early smoke there are cracking sounds ,evolution of
gases and smoke ,some oily liquid vaporizing and even catching fire , a sudden
jumping up of flames and emission of much heat radiation at around
1000 degrees centigrade.
The flames show the boundaries of a
violent chemical reaction between billions and trillions of atoms . In
that heavy heat chemical combinations and chemical dissociations take place at
explosive speed between the trillions and trillions of particles. Molecules
break, atoms come out . In that great heat of around 1000 degrees centigrade
atoms again combine in new groups forming new molecules —all at explosive
speed. The sequence may be different but all following actions( chemical
reactions at atom level) take place
(1) Hydrogen atoms and carbon
atoms come out from trillions of hydrocarbon molecules after completely
breaking the molecules.
(2) The trillions and trillions of
oxygen molecules (available in surrounding air which is heated up and
sucked in ) break up at the high temperature and the extremely active
single oxygen atoms which come out search for new partners
(3) Trillions of the single
oxygen atoms combining with carbon atoms to form CO2 molecules at explosive
speed. Trillions more of oxygen atoms at equally explosive speed combine
with hydrogen atoms to form H2O in the form of very hot water vapor which
escapes into the cool air.
AN INTERESTING FACT
For these high temperature
reactions to occur ,we have first to create at least a small region of
very hot gaseous environment of about 1000 degrees centigrade temperature
i.e. a small flame. We can do this with he of a burning match, a lighted
candle ,a burning torch made of a dry piece of wood or a burning oil
soaked cloth etc.
Ordinary combustion reactions take
place in air even in the laboratory environment(with of course the 1000 degree
centigrade flame available). When we bring the “fuel”(the substance to be
burned) into this region of fire the reaction called combustion starts and
continues till all the fuel is consumed. The oxygen gas in the air is the
“oxidant” or fire starter.
Soon the reacting substances
(hydrocarbons as “fuel” and oxygen gas as “oxidant”) will themselves
create a far bigger region of fire at around 1000 degrees centigrade and make
the fire a self-sustaining and ever growing fire until all the fuel is
consumed. The colossal fire now created may be thousands of times bigger
than the flame of match stick /candle /burning torch. Since it is a heat
emitting chemical rection , it is called “endothermic”.
HYDROCARBONS
We see that the fuel is almost always
an “organic chemical compound” which may be a gas, a liquid or a solid. Here we
have to know a little about the branch of chemistry called “organic chemistry”.
It deals with study of “pure and primary chemical compounds”
existing in bodies of living beings and in the remains of
living beings.
There are millions of such very pure
chemical compounds in nature (connected to living beings or their dead remains)
and they are called “organic chemical compounds”. All these organic chemical
compounds are manufactured in bodies of living beings from other
extremely pure primary chemical compounds called “Hydrocarbons” .
“Hydrocarbons” are the primary pure
substances in organic chemistry just like “elements” in inorganic chemistry.
Interestingly hundreds of purest hydro-carbons can be extracted
from crude petroleum. Some of the hydrocarbons are pungent smelling gases
like methane, some are highly inflammable and strong smelling oily
liquids like petrol, benzene and kerosene and some are soft odorless white
easily melting solids like paraffin wax. Crude petroleum is a easy source for
hundreds of pure hydro carbons. Due to the enormous heat and pressure deep
inside earth all the organic matter dissociates over millions of years into
mixtures of hydrocarbons the most primary organic chemical
compounds.
There are hundreds of thousands of
other organic compounds which are produced by chemical reactions with other
organic and inorganic compounds within bodies of living organism both of plant
and animal kingdom.Crude petroleum is a thick oily liquid with a
petrol-like smell or smell of sulphur and black, dark brown or dark
reddish yellow in color. It is a highly viscous liquid accumulating for
millions of years in rocky basins formed between underground rocks .
Now coming to the chemistry of
hydrocarbons ,it is to be mentioned that the hydrocarbons are made of
only two elements carbon and hydrogen. All the millions of organic
chemical compounds in bodies of living beings or in remains of living
beings were all created after hundreds of very slow chemical reactions
starting with the primary hydrocarbons.
The above introduction about
hydrocarbons is given to stress a very important fact. All the millions of
organic substances — like wood,leaves,oils , milk ,blood,
flowers,fruits cotton,rubber,plastic— are mixtures of hundreds or
even thousands of organic compounds.All the “fuels” participating in
natural fires have their origin ,even millions of years ago, in
hydrocarbons. Thus in combustion reactions in nature trillions and
trillions of hydrogen atoms and carbon atoms are released to react with the
trillions and trillions of oxygen atoms released by the oxygen molecules
when we start a small fire to create a 1000 degree centigrade hot region—using
a lighted match or a lighted dry stick or a burning oil-soaked cloth.
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