S8-SCIENCE OF HEAT ENERGY-WHAT IS FIRE?

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.